Some people believe that. If heaven's so great, why not expedite the process to get your loved one there?
My comment about killing everyone was a joke but to present the question without your tonghue firlmly planted in your cheek, and I detect none fo that, means you've really gone far afield.
Addressing the question directly: The simplerst answer is that murder is prohibitted by the decalogue.
But this does nothing to eliminate the selfishness of one's desire to get to heaven.
Why is a desire to ge to heaven selfish? You've simply declared it to be true and offered no rational to support your claim.
How do you address the notion that if one is singlemindedly and selfishly driven by a desire to get into heaven the best way to do that is through lioving a sin free life and doing good deeds. Presuming this person lives such a life wouldn't we want as many people expressing that type of selfishness as we can find?
AZnd many christians claim selfishness is the ultimate/original sin.
No, they don't. Original sin is the state into which we are born; not an act. We are fallible beings destined to sin. Selfishness can constitute a sin in itself that is a result of original sin but is not original sin itself.
As for the ultimate sin the NT declares the only unforgivable sin to be blasphemy against the holy spirit.
By that logic, the desire to get to heaven is sinful in of itself.
Well, because that logic started with a false premise(s) we can disregard it entirely. You may wish to look these terms up and find out what they mean before trying to dicuss them.
And with that, there's no logical need for discussion with those people.
You're half right.