Can I decide not to take my roommate, who is without a car, grocery shopping?
What's your reason for not doing so?
I don't want to waste time.
If your roommate needs help, you should help him. He depends on you.
My time is more important than taking him to the store.
That is a mean thing to say. It is rude to not want to help others in need. Maybe you can let him go with you when you go grocery shopping that way it is easier on you.