Alternative Character Interpretation: Bellamy's revelation that he's still angry and traumatized about the events at Mount Weather in season 2 call into question if he has always distrusted the grounders even during the 3 months after Mount Weather but he was holding it in because of his relationship with Gina and to maintain the truce and the death of Gina brought it back to the surface and also the fact that trusting Echo led to her death he felt probably confirmed his belief that grounders can't be trusted and that peace with them is impossible.
Another interpretation is that he's suffering from PTSD from the events at Mount Weather and the war with the grounders before that and it is whats fueling his decision making and why he's being irrational.
