Hey onget, and welcome to the forum.
At the moment, the main rewards for killing a summoner are the significant amount of experience points, and the opportunity to recruit their minions, which often come pre-loaded with useful abilities their former master copied to them.
I agree they could use more of a reward though, since the experience points is what you'd expect to get from any tough fight, and the other result is fairly random. The Relic Upgrade system may offer me an opportunity to do something here, or I could have them drop Credits perhaps. Haven't decided for sure on this quite yet.
Thank you for the feedback, and I'm glad you're enjoying Demon.