It could simply be that what he says he wants to be true, even if inside he still wants and likely does blame Heath. For Lily's sake he's been trying to move on, and saying its not anyone's fault is him honoring his wife's final words and wish to not blame anyone, even if Heath is to blame.
Basically, he's trying to convince himself that what he says is true by saying it out loud. I bet he still hates Heath.