No, trying to rationalize his point of view is defending him. Ignoring the obvious dickery and trying to be ambivalent about it all is defending him.
Let's look at the facts, not only did he want to execute us without trial (dick move number 1 since he didn't actually witness anything other than an explosion), but he killed Mieren without a moment's hesitation. Could he have disabled her and left her alive? Yes. Could he have taken her prisoner instead of cutting her head off? Yes. So, all that time together meant nothing. The friend that moments ago everyone was broken up about only being able to find her arm, suddenly deserves death for asking for a trial and liking us rather than him.
It'd be one thing if they all saw us do it, but they didn't. Knowing what we know, it does seem slightly more justified, but imagine that it wasn't us we're talking about. Imagine that this was some other guy who chose not to betray them, fought the demon and almost lost, but in the moments before the demon prevailed, he felt the irresistible urge to throw his amulet at him - no doubt a fail-safe of the deities who gave it to him. It exploded and the same scenario happens.
Would Kyle (who doesn't know what happened either way) still want to kill this hero? Yes.
Would he still murder Mieren for defending an INNOCENT man? Yes.
So what we are looking at is a group of people who would not give two of their own the benefit of the doubt and would straight up murder them both based on flimsy superstition and jealousy.
They are assholes. Kill. Them.
and I want to find that girl's severed head and put it in a jar. Eventually we need a spell to raise her into a passably attractive female head.
Reanimating Mieren into a passably attractive female head would have been far easier as an undead lich lord.
yeah, I know, but let's just say that if we were a skeleton, we wouldn't have the, uh, "equipment" to take advantage of such a scenario...