I think a lot of the "planning" they did was after the fact.
He may have been wanting to harm KB for months but I think the actual event was due to some psycho rage. The fact that blood was so widespread, the fact that much of their "plans" don't make a lot of sense to us, and they both left things "undone" and stories are a bit scattered...all that tells me he lit up in the heat of anger and decided then and there he would do it.
The planning and resulting cover-up was haphazard and seems CYA vs. planning. But they didn't do a very good job of CYA'ing.
Some keep looking to tie it (their actions) all up in a package that makes sense. No sense at all to this, just a lot of mental/emotional sickness going on.