How can I get the worst reputation possible?

The following conditions will greatly impact on NPCs stance towards you.

Vampirism

Vampires are hated and feared by the general population and will be attacked on sight if their true nature is discovered.

• Stage three vampirism: most NPCs will not interact with you if you initiate the conversation.
• Stage four vampirism: nearly all NPCs become hostile and will attack on sight, including spouses and followers.

Lycanthropy - Beast Form

Almost all NPCs will become hostile (exceptions include other werewolves, Maiq The Liar, Melka, members of the Dark Brotherhood, Paarthurnax, followers, housecarls, and members of the Thieves Guild [verification needed], but not your spouse, unless your spouse is one of these)

Note: You have to choose to be either a vampire or a werewolf.

High Bounty

Comitting any crime causes the Hold guards to attack you.

Bounty Collector

If your bounty is above 1000 gold, you may be met by a Bounty Collector.

Sacred Chickens

There are consequences for killing a chicken. Even though the bounty is as low as 45, the townsfolk will kill the violator if they get the chance.

Sheathing the weapon has no effect on Hold Guards. You better run.

Source

For some reason chickens are for Skyrim what cows are for indians. Not delicious but sacred.