Chris has a long history of choosing to do the worst possible thing in any given situation. The signs of his creepy mommy attraction were to be seen long before even Bob's death. I have little doubt he did the deed.
The question that remains to be answered now is how much Barb was involved. Is she really as vegetative as she appears to be, or has she sunk to using sex as another tool to manipulate her son? It's not entirely impossible, given the timing, that she deliberately started this right before Everfree, because she has a pathological need for control, and cannot stand him leaving her for that long.