I think pro-mortalists genuinely believe that (an early as possible) death is the best solution for any living creature unfortunate enough to have been born here.
That said, even the most pro-mortalist members here still (thankfully) respect the right to choose I imagine. I doubt they go around telling everyone they would be better off dead!
I hold views similar to it but I'm not sure how this view even works online nowadays and even backadays (not sure if that's a word), I don't really like entering philosophical forums/subs, whatever, I hold my own views from what I learned, obviously they have existed before and were invented by others before me. I don't have the power to choose for others, their choices are their own. Many people might agree with this view of life even if they enjoy it, because they can objectively see others life from their point of view. Someone could have everything they wished for and still end up saying death is better than life and not being born is better than both, just like getting raped and tortured doesn't need to happen to you to know it's preferable to not get raped and tortured or for it to end as soon as possible.
Basically, this view has many branches, you don't owe anyone anything, if you wish to die, you can die, if it's your wish. If you want to live and help animals/humans and reduce their suffering (since you can't kill them all painlessly in one micro second) you can do that (If you wish, you are not forced to even if you are aware, you are not owed anyone your life). Murder under pro mortalism is not helping the cause, because it isn't a choice for the people who get killed and neither is it just, you could have killed the 6 happiest people that were objective thinkers who studied views like anti-natalism and pro-choice in suicide and could have in 50 years made 1 billion people able to critically think and kickstart an era where euthanasia is seen as a good thing, in the end those 6 people would have given the choice of being able to die to many more people. Although the murders don't happen because of a will to propagate pro-mortalism, they happen because the people are egoistic and sadistic, obviously.
Someone who actually believes in something like pro-mortalism philosophy and really wanted to make a difference, would educate people on critical thinking so that societies might allow suicides at one point and talk more openly about how life is shit! Because the goal isn't sadism or exerting control over others, the goal is for everyone to understand that death and not being born is preferable to life in a lot of circumstances and that many are forced to exist against their will, by peer pressure, self preservation instincts, etc. The goal of it, should always be to teach people that they have a choice and if they want death, it's theirs and that it's not a BAD answer, it's an answer and most likely the best answer for those who were born, not all of them, but too many of them.