Why shouldn't I murder all the things?
Playing through Dishonored, I sort of decided that my first go through of the game I would just murder every enemy in sight. Apparently, the game really wants me to NOT stab everyone in the face, and is punishing me for leaving a bunch of dead blokes scattered about Dunwall.
I was given a vague pop-up after the first mission and it said something along the lines of
"Because you killed so many people there are more city guards, rats, and weepers in the city. Also, your ending has become a bit darker."
I'm curious what exactly this means. I have noticed more enemies around, which seems to be obvious. However, will the amount of enemies around increase after every mission that I go Facestabber mode on? And much does it effect the game's outcome?
Violent solutions to problems and a high body count increase the level of Chaos in the city.
When the city is a violent chaotic place, people react accordingly. There are more guards and more traps. There are more rats to take advantage of the abundance of food. Essentially, if you signal to the game that a fight is what you're after, then a fight is what you'll get. The final mission, in particular, is dramatically altered by the level of chaos you've engendered or avoided up to that point.
Additionally, Dishonored has several endings, and chaos is the primary determinant of which of these you'll see.
I did two playthroughs: mostly stealth (less than 20 kills), then mostly murder (less than 10 survivors. I was thorough). I did not find the quantity of rats, guards, or guard security mechanisms to be substantially different between the two playthroughs.
In the murder playthrough, there was one place where I felt that there were more weepers than normal, but that could just be my memory.
A few things that I noticed:
- Killing all guards in an area sometimes triggers another wave of guards to appear nearby. I'm not sure if the first set of guards need to have been panicked to cause this. In the Golden Cat mission, in the square with the Art Dealer's house, I found the guards would respawn up to 4 times.
- The only situation I've seen where guards seem to perpetually respawn is in the final mission, prior to boarding the elevator. Then again, I may just not have killed enough. I was getting bored and low on health.
- In the Golden Cat mission, where you get the "Chaos causes more rats and security" message, there is a guard booth nearby the sentry tower. Inside is a message that says "Due to the recent death of Captain ___ in Holger Square, we've put a sentry tower here". I suspect that this sentry tower might not appear if you're careful in the early missions.
- Don't expect any new dialog until the part where you get poisoned. Emily will paint a different picture of you, but that seems to be it.
All up, I don't think there's really any difference between the playthroughs, unless you want to see different endings. The psychopath gets 2 endings, depending on last few seconds of the game (easy to save before it and try both), and AFAIK there's only 1 ending if you let everyone live.
I've finished the game on low chaos and the first 3 missions on high chaos, and I can tell you that there are a lot more things that change in addition to what's been mentioned above.
The world becomes increasingly more of a mirror of the monster Corvo becomes, even in earlier missions:
1. Dishonored
Not much difference here except that the announcer places more emphasis on the seriousness of your escape with several brave state officers dead by Corvo's hand. Corvo "...must be captured or killed at any costs."
2. High Overseer Campbell
Low Chaos:
An overseer in the workhouse fears he has the plague and begs his brothers to kill him. After trying to talk him out of it, they reluctantly oblige.
High Chaos:
The same overseer is much sicker, but tries to hide it. He tries to reason with his brothers, but they brutally stab him in the back and make plans to dispose of the body.
3. The house of pleasure
Low Chaos:
Morgan retires to the smoking room. Slackjaw notes that Corvo does look like a man out for murder, but is more cordial in his initial address.
High Chaos:
- If you poisoned the still, the thugs will be sicker, and a weeper ambush of the infected civilians awaits the distillery as you leave.
- Slackjaw calls Corvo a villain, but one who he has some work for. (WARNING: If you've opened the weeper cages before talking to him, he attacks you on sight, and his men join in)
- The Announcer adds that due to criminal activity, Holger square is under lockdown and anyone who's not a guard will be considered hostile and executed on sight. There are more guards there, and a few respawn if you leave the map.
- Morgan retires to the steam room
- Many more weepers and rats throughout the level.
- Granny rags tells you a dark story of a bodyguard that escaped and won't stop until everyone is dead... "Dead HAHA!" but invites you to retell the story, and "change up the ending"
Why shouldn't I murder all the things?
Mainly because it will increase Chaos levels.
The Dishonored wiki, on the Chaos system:
Changes in the Chaos system may result in subtle in-game consequences, such as more rats appearing in an area or slight differences in dialogue, to more noticable consequences, such as more guards appearing on patrols or characters refusing to interact with Corvo or rewarding him, depending on what they think of his actions.
If high enough Chaos is created, bedlam ensues: NPCs begin to attack each other, rats swarm the area, and the world through which Corvo moves becomes an altogether nightmarish place.
The developers emphasise that the Chaos system is not a morality metre, but a stability metre. It is meant to highlight the plight of Dunwall: its people are no longer concerned with right and wrong, but merely with holding on to the little certainty that still exists.
I noticed that on a high chaos playthrough, NPCs like some Survivors will alert hostile, armed NPCs, if they detect your presence. While on a low chaos playthrough, some Survivors will be friendly to you or won't mind your presence (no detection meter), and won't alert anyone.
Also, there is actually a significant difference between high chaos and very high chaos (not shown in the ending):
I just finished a 3rd playthrough where I went lethal+stealthy. As a result, I had a moderate number of kills - the first few missions said I had Low chaos, but it had changed to High by the time I got to the last mission, since I went on a bit of a rampage with the assassins. On "very high" chaos, Samuel is a total dick to you and warns guards by firing into the air. On my playthrough, he simply said he was disappointed by how "brutal" I had become, and drove the boat away in silence.
Source: Reddit /r/dishonored: "There IS a difference between "high" chaos and "very high" chaos, just not in the ending"
Also, see this answer on another question about how the Chaos system works.
Even in low chaos there are variations in endings... if you don't save some person (not telling) he wont appear in the ending rather someone else. If you go undetected then your posters show a ? in place of face... etc.