What is the difference between a kill steal and good teamwork?

Whether a kill is a "kill steal" or not is very subjective. The main time this really matters is if it is a support player who might get the kill or if a carry is currently behind, then you want to try as hard as possible to not get the kill as the support player and to allow your AD carry to have the kill.

However, the game is fast paced and many champions have tricky escape mechanisms with cooldowns you can't always keep track of. For this reason "ks-ing" is sometimes referred to as Kill Securing where everyone focuses on killing the enemy champion before they can escape without concern for who gets the kill credit.

In a solo/duo queue game it's usually quite obvious when people are trying to actively Kill Steal as they will save some skill with Burst damage to make the last hit with. This is generally not considered good teamwork or polite.

Honestly brah, don't worry about being a kill stealer if you aren't in a serious game (normal games with random people). Only time kill stealing should be an issue is in ranked games or attempt to play at a high level. Even then, shit happens and if you think that your team mate will not be able to pick up the kill; then you need to make sure that punk dies.

and if your team mate gets mad at that you just say "you mad bro?"

My friends and I have two meaning for "ks." there is "kill steal" and "kill secure."

when your teammate is in a 1v1 and they quite obviously have the kill or have been doing loads of damage and just need you to throw in an exhaust, you really shouldn't take the last hit because that's a kill steal.

Another popular form of KSing in solo queue is when the support in bottom gets the last hit during a kill. While some AD carried are quick total this a steal because they output most of the damage, sometimes the support is unsure if they are going to get the kill, so they just keep auto attacking, or they really did just get the last hit from luck.

One example of a "kill secure" is when the mid lane is fight one on one and they are both low, then one side starts going back and the other is two low to tower dive, so the nocturne on your team swoops in to get the last hit. It was obvious you weren't going to get the kil, so he just came in to"secure" it for the team.