Well...yes, Joseph Smith at least taught that Christians do go (to hell) ... at least if they reject Joseph Smith...although they do define "hell" distinctly:
Questions put to Joseph Smith: "'Do you believe the Bible?' [Smith:] 'If we do, we are the only people under heaven that does, for there are none of the religious sects of the day that do'. When asked 'Will everybody be damned, but Mormons'? [Smith replied] 'Yes, and a great portion of them, unless they repent, and work righteousness." (Teachings of the Prophet Joseph Smith, p. 119).
You can see reinforcement of the above by later Lds leaders if you look at the link below:
See sections 2, 3 & 4 of this thread If Mormonism is 'anti-Christian,' then how can it be considered 'Christian?'...
Note here that Mormon leaders (like Brigham Young + Lds "apostle" Bruce McConkie) & reserved for those who don't accept Joseph Smith as the "passport" -- the official "consenter" -- of whether we as Christians can enter therein...as titles like the "anti-christ"...just for rejecting Smith.
Caveat: Mormons define "hell" as less than "eternal"; Smith defined "damnation" as "damned up" -- which, no, etymologically "damnation" NEVER meant "damned up"...so even when Smith consigned Christians "to hell"...Mormons can try to claim that wasn't eternally." (Still, Mormons say Christians WILL NOT be able to live eternally with Heavenly Father...no matter what they do or don't do...as long as they don't "convert" to temple Mormonism)
I'm not sure that Mormons do believe that non-Mormons are all going to go to hell, but plenty of non-Mormon Christians believe that people who don't believe as they do will go to hell, so what's the problem there? [x, post #146]
As for the last half of your statement, you're essentially asking, "Isn't this a kind of a level ground here? If Christians claim certain people might be hell-bound, what's wrong with Lds doing that?"
Allow me to clarify a few things: Christians don't simply claim that orthodoxy is THE way to heaven...tho we DO say -- that being in a right relationship with THE True God -- and His Son -- is vital...
For Jesus Himself said that...so it's NOT original with us:
3 Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent. (Jesus, John 17:3 during prayer to His Heavenly Father)
Likewise, also NOT original with us, is this statement from Jesus:
The Christian gospel: "I am the Way, the Truth, and the Life. No man comes to the Father but by me." (Jesus Christ, the Head of the universal church, John 14:6)
Now...how is that distinct from Mormonism? Well, instead of Jesus Christ being "the Way, the Truth, and the Life" -- the Mormon church as made either...
... Joseph Smith" the passport, the consenter, the way...
...citations for that are found at: If Mormonism is 'anti-Christian,' then how can it be considered 'Christian?'...
...Or, it has made itself -- the Mormon Church -- as the way, the truth and the life...and that comes directly from Mitt Romney's father's cousin, Marion Romney, who was one of the top three hierarchists in the Mormon church for a while:
"This Church is the ensign on the mountain spoken of by the Old Testament prophets. IT is the way, the truth, and the life" (Conference Report, April, 1961, p. 119)]
But the question of just what "Mormons" or "Christians" or "Jews" or "Muslims" believe is also complicated by the fact that across time, you won't always find people believing what their co-religionists claimed to believe centuries ago.