Actually I always assumed Dilan became against experimentation and science itself because of what happened to him and his fellow scientists when they puttered around with it too much. Not to mention the experiments they did possibly on other people before it went haywire on themselves; he could of easily done these experiments and felt guilt for their actions but continued on.

The history was not given for the original six in the game so often people are just left to rampantly speculate.

I was thinking his hatred for Xehanort dwelt from his own mistakes because of his faulty judgment which lead to him losing his fellow comrades, see a ton of pain, identity, and being completely consumed by the darkness. That was my interpretation at the very least.