So you want to your character to be insane? Look, you can't just run around screaming about shit following you or people turning into monsters and whatnot, you need some reasons. Here are a few good tips to make a convincing backstory for a batshit playstyle.

GM: You enter a local tavern...

Barbarian: I sit at the bar and order the wench to serve me an ale!

Fighter: I set my shield on the table and inspect my weapons from after the last battle

Rogue: I saunter up to the gambling table and casually grab an ale and start to quaff it

GM: is it your ale?

Rogue: no...

GM: The grizzley old man next to you begins to glare with a type of bloodrage you are only used to seeing in an angry kobold...

Rogue: I tell him not to worry, if I win, I'll buy everybody a round!

GM: Roll for bluff

Rogue: ...1...

GM: The guy kicks you in the nuts. Hey Cleric, what are you doing?

Cleric: I've sat down at the bar and immediately started drinking a LOT of whiskey

--Time passes--

GM: so you guys are completely hammered by now...

Barbarian: I stumble over the bar and grab the nearest wench, trying to drag her up to a room

Fighter: I've gotten into a headbutting contest with the local champion

Rogue: I've ended up buying everybody a round...and now I'm trying to pickpocket somebody to try and get my money back

Cleric: I slowly pick up my head from the bar, and start screaming quite loudly about how everybody has turned into kobolds

PAUSE!!!

So you want to your character to be insane? Look, you can't just run around screaming about shit following you or people turning into monsters and whatnot, you need some reasons. Here are a few good tips to make a convincing backstory for a batshit playstyle.

1. Have your character suffer some type of trauma early on in life. For example, maybe your family was murdered by kobolds. This could be a good start down the road to insanity. But seriously, people get killed all the time in D&D, you need something really messed up to push you over the brink. So in the case of the cleric here, perhaps the kobolds skinned his family and wore their faces as battle masks, and he came back home just in time to see them walking around with the countenances of his loved ones.

2. Your character is obsessed with things of a demonic nature. Demons are always insane, or at the very least insanely scary. If you hang around with them a lot then it stands to reason YOU are going to be insane too.

3. Maybe your character just simply takes a lot of head wounds.

There are a variety of ways you can make your character become insane. Just keep in mind that in order to make the most of your insanity you need to incorporate whatever made you insane into almost every thing you do. All your actions should be motivated by whatever is making you crazy. If you can do this, then your experience as a crazy guy should be an entertaining one.