The leftist position against private ownership of firearms comes directly from their core values:
1. Progressives are collectivists, they reject individualism. The modern progressive has no moral qualms about sacrificing individuals for the "common good".
For example, suppose a woman believes her crazy, jealous ex-boyfriend is likely to try to kill her. She goes out to buy a gun, but she lives in Commiefornia where the progressives there have instituted a 10 day waiting period. During the waiting period, the ex-boyfriend breaks into her place and strangles her to death with her own pantyhose. Leftists will view her murder as a necessary sacrifrice for the common good.
2. Progressives are socialists, they want the state to control production and distribution of guns, hence the mountain of regulations pertaining to which guns may be sold and to whom and under what circumstances. Two hundred years ago justice John Marshall said "the power to tax involves the power to destroy", which is of course true. But the power to regulate also grants the rotten government the power to destroy.
3. Finally, progressivism is synonymous with statism. They reject the very idea of private self defense, they want you dependent on the government for protection (and everything else for that matter).
But what about liberal gun owners, don't they exist? Sure, they exist. Most of them are environmentalists as well, while they drive around in their gas guzzling SUVs and have a personal consumption expenditure of over 40k per year. Progressive values are to be imposed on other people, not themselves. That's why after they ruin a particular state, they move out of it, to go ruin another.