Personally, I do trust Galanoth. Granted, it's not a full level of trust one would give a true friend, but I have hopes that he won't turn on us- the Hero/EDoT- without good reason. I'd class a good reason as going on a rampage or if we are no longer in control of ourselves.
I know the timelines are different, but in other games (if I remember correctly) he started out as hating dragonkind in general after losing his parents and home to Akilroth*. This extended to Dracomancers as well. Eventually he ended up becoming more open to others' opinions and considering alliances.
I'd consider the slaying of the EDoT to reset the timeline as a desperate act; one that felt like the 'only' choice. And like Thulsa Doom said, the EDoT allowed Galanoth to do this. If he knew his part in that situation, I somehow don't think he'd consider doing it again unless the first times' circumstances repeated themselves, due to the EDoT's importance. Basically another 'end-of-the-world' scenario.
Although, in AQ3D... Galanoth is portrayed as being obsessed with trying to get revenge on Akilroth to the point of potentially endangering others AND wishing to hunt every dragon into extinction. He even states that: "If you are not with us, you are against us". He actually reminds me of Duncan from the Lair midweek here. Who, funnily enough, was kicked out of the Dragonslayer Order, and refuses to speak with you if you help Ferzana...
*Aren't we eventually going to have a storyline about Galanoth and Akliroth in AQW? I am quite excited for this, especially if his history from other games is used at all. Then we'd have a real answer to this question.