As an atheist who did indeed point out the murderer's connection to the SBC, here's my take.
From the initial reports, it seems he was motivated by guilt over sexual sins.
Jesus himself addresses this situation.
27 Ye have heard that it was said by them of old time, Thou shalt not commit adultery:
28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
29 And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
30 And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
Who is guilty of this sin of lust? The woman or the eye of the beholder?
Who needs to be chastised? The woman or the lustful one?
So I mean, we can just say, "Hey this guy was a terrible Christian. Murdering people is not really Jesus' style."
And yet by all accounts, he was a longtime church member. Underwent a second baptism a couple years ago to rededicate himself.
So could the message he got from his church have been the problem? I don't know how much time the SBC spends on that particular passage in Matthew, but as an outside observer I don't hear that message much from American Christendom. Apparently the focus of the last Sunday sermon in his church was about the apocalypse, Jesus returning soon, and the sinners getting what's coming to them with the wailing and gnashing of teeth. There's no evidence the murderer was present for this sermon, but as an outside observer, I do get this impression that this thread of apocalyptic and nationalist muscular Christianity has become quite common now that the political has so become wedded with the religious. People are getting radicalized from the pulpit. This horrific outcome might (I say might) be an example of that.