I dip my flatmate's brush on the toilet bowl with my shit
I'm very disgusted, why would you do that?
Because I don't like him.
Whether you like your roommate or not, you're still expected to be respectful to them. If you don't show people respect they will not want to be around you.
But why? He doesn't give a shit about me.
If he doesn't care about you, he is still expected to be respectful towards you and your property and you should do the same. It's not okay to dip his toothbrush in the toilet bowl as it is very unsanitary and you can make him very sick by doing so and you will be too guilty in the end. I suggest you clean it and make sure you do not do it ever again.