Quote: Why are you offended? The religion forum is located in the category called "The Hot Seat."
It is offensive not because of its content, but its context. You are using a tragedy to try to levy an argument (and not really a relevant argument at that). It would even be in poor taste if someone tried to capitalize on the tragedy to make an argument for or against gun control -- and that argument is at least somewhat relevant. Just a couple days ago, a dozen people were murdered by a stranger and instead of showing respect to those lost, you saw it as an opportunity to gain some points for your ideology. Do you see why that might reflect badly on you?
It actually reminds me of a conversation I had a few weeks ago from a friend of mine. I hadn't seen him in years, but he was a good friend in college and he shared very similar political and religious ideologies as you do (from what I know about you both). However, he expressed how he was coming to an epiphany from his experience with a church he attended for years. As he put it, he was feeling disillusioned because "it seems like they are only concerned about peoples' souls, and not the people themselves."
The sentiment seems to be reflected in your argument as well. There was a tragedy which should as a society bring us together to mourn, support their friends and families, and show respect for their loss. Instead, you immediately took advantage to be divisive and try to score religious points.
That is what bothers me most -- it is a sort of disrespect that makes the Westboro Baptist Church and politicians who try to capitalize on tragedy so infamous.
However, your argument is also very weak. It is based on speculation, which is never a good way to start an argument. If your premise is ill-supported and very possibly false, an alarm should go off in your head that says, "I don't have much of an argument if this is what I'm depending on."
In this case, I hope it has served as an opportunity for you to learn that lesson. As it turns out, your speculation (that the serial killer was an atheist just because of his occupation) was wrong. To the contrary, he actively participated in church. As a result, your whole argument crumbles because you made this irresponsible speculation.
Your assumptions didn't end there. Your whole argument used ill-supported assumption on top of ill-supported assumption. Doing so is "specious reasoning" as I put it before. Even if I ignored the distaste in your capitalizing on tragedy, your argument is just plain-out weak... and it turns out, quickly debunked.