Oh, you misunderstand: I totally leave it up to the Lord Jesus Christ! As a believer in Jesus, I read what He says, I believe it, I affirm it. I would be an arrogant fool to do otherwise.
And He Himself affirms OT teaching that Mormonism denies — that there is but one God, and He has no equal. And He says through the apostle Paul that anyone, angel or human, no matter how nice — and some of the nicest folks I know have been Mormons — who perverts the Gospel as Mormonism does will be damned.
For me to say other than that those who embrace the teachings of the charlatan Joseph Smith are still under God's damning sentence would be to be arrogant and disbelieving towards God, and indifferent and unloving towards my fellow men.
Secondly, as a Christian, I certainly do not believe that "others can just say 'I'm saved and believe in Jesus Christ' and all their sins are forgiven them, regardless of how they live after that." No Biblical Christian believes that! If it is a heresy to say that we are saved by works (as Mormons do), it is no less a heresy to say we are saved by words. The Bible is crystal-clear on that fact.
We are saved by grace alone through faith alone; and the faith through which we are saved never remains alone. It shows itself in works which are the result (and not the cause) of salvation (Galatians 5:6; Ephesians 2:8-10). Anything that denies either factor is a damning heresy, whether Mormonism or libertinistic antinomianism.
If you do not repent and believe in the true, living Jesus Christ, you will be damned for your sins. Sin includes refusing to do what God says, and refusing to believe what God says. In any of our case, if we do as you do, and trust to any degree to our works, we are lost. I would be no less than you.
As a believer in Jesus, I read what He says several hundred years' worth of pseudo-Christian demagogues say that Christ said, I believe conform to it like a good little minion, I affirm it (so they won't hurt me). I would be an arrogant fool show some actual guts and fortitude to do otherwise--and I just don't got what it takes.