in Buddhism, would be murder. Guaranteed trip to the hell realms.
For Christians, it appears to be putting anything else ahead of God. Correct me, if I'm wrong, please.
In the OT, having no priorities over God was number one in the top ten.

I would assume that this is because this would insure that one was always thinking and prioritizing in a correct manner.

Keep in mind that this was long before Christianity was a religion, so it really wasn't a "Christian" sinning issue.
My wife and I discussed this topic of what is the worst Buddhist sin, and she disagrees that it's murder.
She thinks "wrong view" is a more severe sin.
She also reminded me that Angulimala, who killed one hundred men, became a great arhat, and Milarepa, who kill an entire village as revenge with black magic became a full realized Buddha.