I used my roommates toothbrush to clean the toilet
Why would you do that? It is gross!
I thought it would be hygienic, to use my roommates toothbrush to clean the toilet
I mean, it is very rude to use other people's things without them knowing, and also using a toothbrush to clean a toilet is extremely unhygienic and rude. You should apologize to your roommate and buy them a new toothbrush. Otherwise they will learn about it and will want to you to move out in anger.
They will not give a shit and they have many other toothbrushes. I'm going to tell then what's up.
You definitely should tell them and throw that brush away. How would you feel if someone cleaned the toilet with your toothbrush?