It's better characterization and drama if the char is acting the way they are on some level becasue of their own feelings, and not entirely due to a foregin substance that messes with their heads. It makes for better development in that they have to accept that it was their own feelings (onsome level) and that they could have done something about it if they hadn't given into the darker impulses. That's also why Raimi had Peter be able to take off the suit at first, to portray it as something he could have ogtten rid of or not used, but he enjoyed the power and thus willingly donned it when he was hurt.