Generally speaking, it is also possible that human logic and/or story telling as we understand it may not apply to the spirit world. What we as three-dimensional, mortal beings may consider a 'motive' for behavior may not be the same as what an 'evil' spirit may consider. There might be layers to our present world of which we are not aware that may be beneficial for evil spirits to draw upon/explore. Or maybe the spirit isn't 'evil' at all, but something about it might upset the balance of this dimension with us trying to interact with it or we fucked up trying to bring it in via a 'one-summoning-spell-fits-all.' Maybe the spell didn't complete the transition and the 'evil spirit' is simply the spiritual equivalent of a chicken running around with it's head-cut-off just causing fucking chaos all over the place. Or maybe the summoner's deep, dark unconscious is what is projecting on the spirit in this plane and that's what is really causing havoc. Maybe it's not even a 'spirit' at all. Maybe its some aspect of ourselves given a new form.
Maybe we're part of a great game. Maybe it's all just random. Maybe it's all just in our heads. Who the heck knows?
It's still fun to speculate though.
I'm personally fond of the 'more than we know' theory---that we're just peering at our world through a teeny, tiny peep hole relative to our human minds and thinking we know all the answers when really, there are layers upon layers of the world that exist at frequencies of which we are blissfully unaware. We might be co-existing with other frequency beings and not even know it. A stacked multi-verse theory, anyone?