It seems to me that Millard wanted to hurt/kill Babcock for quite some time. The "I'll remove her from our lives" message was from April 2012 and he didn't kill her until July. He took his time acquiring guns and the incinerator, and once he had those things he couldn't wait to use them on Babcock. IMO, given the evidence so far, the urgency to kill Babcock was driven more by his desire to kill her than a perceived need to kill her. He killed her the same day the Eliminator cleared the border. IMO he couldn't wait to use his new toy. He probably freaked out when he realized it didn't just work out of the box.
Also, with Bosma; I never saw any good reason why they would kill him, other than they could. Sure, they had the truck to gain, but murder is not a necessary part of vehicle theft, IMO.
Wayne was different because they couldn't simply kill him and make his body disappear. That would have held up the passing of his estate. Instead, they had to stage it as a suicide so that Dellen could receive his father's wealth quicker. But, again, there appears to be an element of sick entertainment in Wayne's death too. Remember Dellen's photo on Steam (online gaming site) with the bloody eye? It was uploaded in November 2012, the same month his dad died by a shot to the eye. IMO he was deriving pleasure / satisfying an evil hunger from committing these acts.