There's another thread about this...
Basically, everyone in the world hates imps. The first time Wyll summons on in-party, all the party members get a -5 attitude towards Wyll (standard popup, but it is short). There are other places you can see that having or summoning an imp gives attitude maluses. If you have an imp summoned when going to camp, Scratch pops a -5 attitude towards you and usually attacks.
