Why so many "saved" Christians are so concerned of others salvation?
Probably because they genuinely care whether or not you, and others, are embracing a gospel that saves, or a false gospel that offers you no genuine reassurance of your salvation.
For example, if you believe in a Jesus who is not both God and Man then you are trusting in a false Jesus that cannot save you and you are headed to hell.
The kind thing to do would be to point that out. If you got on the wrong train headed in the wrong direction, would you be offended at someone who is trying to get you on the right train?
We should be working on our own salvation instead of slandering or mocking others whom we don't agree with.
But you aren't working out your own salvation, you are working on a spot in hell.
I would have you repent and embrace the only Christ Who can save you, the Word, Who was in the beginning with God and Who Was (and still is) God.
The Alpha and the Omega, the Beginning and the End, the Creator of all things.
The ultimate kindness is an open rebuke, hatred would be to say nothing while you continue to ride the wrong train in the wrong direction.