It is indeed wise to consider travel advice, especially when going to a war zone. One should not just denigrate the actions of this man for that though, since he was motivated out of humanity and compassion, one imagines.
Those are worthy attributes. Not having anything to do with the case, I do not know why he did not travel with a group like MSF, and, once imprisoned in such circumstances there will be limits to what diplomatic negotiation can achieve. So it just seems to me, in part at least, that a grieving family are looking to come to terms with his death by assigning blame.
Based purely upon what I have seen, suicide does seem unlikely,but as Zeuhl has said, releasing him would have given Assads regime a PR boost, unless of course he was treated so abominably that his captors felt it was better to silence him.