Isn't the context about the overthrow of the Russian Empire by communist revolutionaries? Not modern first-worlders? Am I missing something here? Why would "foreign imperialists" be relevant to modern first-worlders?
That being said, to actually answer your line of questioning, it is the correct solution to change society while ALSO overthrowing and locking up the oppressors. That may involve the elite dying, but those deaths are necessary. Peaceful reformism and strict nonviolence policies never works – unless you consider extremely high amounts of unnecessary suffering for innocents to achieve comparatively minor goals as "success" (cough cough Nelson Mandela). Even Gandhi and MLK (who took most of his influence from Gandhi), although nonviolence advocates, were well aware that violence is often necessary to achieve a better future, and much of the work they did was to the benefit of violent/militant revolutionaries (although of course they're portrayed a lot more neutered/"deradicalized", as well as the roles of complete compliance to nonviolence being completely overstated while violent methods are hidden away as if they didn't exist, not even to be mentioned).
After capture though, death pentalty is not the way to go, but life imprisonment is fine and they may have a chance to be released later, mostly depending on their status/loyalty. I'm sure a lot of "revolutionaries" would disagree with me though, but I'm not an "eye for an eye" believer... I suppose if you're in a situation where the former imperialist rulers would likely have power to directly cause damage while detained or incarcerated, or they're likely to escape or be "rescued", then it would be justified to chop off their heads or put a bullet in their cranium.
The core issue is that these people (the oppressors/ruling class) can not be rehabilitated, and are likely to stir up considerable trouble and disrupt when they have the opportunity, either in a bid to regain their power, or out of a large feeling of loss that makes them go nuts. You can't always reasonably ensure that they won't try to fuck shit up in the future.
That's just my view, but of course there are people other than me who are just bloodthirsty for vengeance (my opinion is that they're not thinking all too rationally and it's the same mindset as parents that hit/yell at their kids, they're convincing themselves it's for the greater good but in reality it's just attempting to satisfy their feelings of anger). Either way, I see their lives as considerably less valuable than the lives of the people they oppressed, not because they have an inherently evil soul or something, but because they are already too far gone and only can bring chaos to the world.