That's a good point.
I'm not from the US so I can't say I know everything about it (nor have I ever claimed as such).
I guess it's a normal conclusion to reach for the BP considering how they have reason to mistrust local governments.
Which is actually the point of Marx's statement,who the Black Panthers were influenced by, they were officially a socialist group.
They understood that the government is rarely your friend, and that ultimately arming the populace is the only way to keep them properly checked. Of course, the FBI would use this to manipulate their actions and public perception to make them seem like the counterparts of the KKK.
I still don't see how the left has always been lax on gun control.
In this regard you're definitely right. At least in modern times (lets say 1980 onward) large portions of the left in America have generally been pro-gun control, you're correct. But to say or suggest, which I'm not sure that you were, that there isn't a history of armament in leftist ideologies is not correct.