Can I throw away my roommates dinner?
That's mean. Why wouldn't you finish their dinner for them?
I just don't want to eat it.
If you don't want to finish someone's meal, then you should find a container to store it in. Throwing away edible food is wasteful. Save their food and let them know you did it for them and they could be appreciative of you.
Sometimes people take things too literally. I just didn't want to eat it and I'm not saying that I'm going to throw it away. I was just thinking about it.
Good, wasting food is wrong. And it can be more expensive to buy new groceries instead of eating leftovers.