Facebook already bans ISIS propaganda videos, so it's a small leap to assume that they'll also ban Al-Qaeda released videos as well.

They're not illegal, but many sites ban them - I tried to find an unedited copy of the ISIS video with them dropping grenades from a drone a few years back... you often wind up having to go into the same niche news sites, gore sites, or torrent sites which you do to find this video. My options were pretty much 8chan itself or "BestGore" when I looked.

I'll also note that ISIS does a lot more editing and commentary to slant the content in their favor than this guy did. About all this guy did was make meme references... I can see an argument that this is quite different.

That would entirely hinge on the degree to which the content was editorialised. If the video contains commentary and/or editorialising and/or editing that in any way implied that the occurrence was desirable or righteous, then it is terrorist propaganda and should be restricted without hesitation.

If it was an opinion-free, propaganda-free raw unedited video of a newsworthy event that was filmed in a public place, then it probably shouldn't be banned—though publishers should still act responsibly if the material contains deeply offensive or distressing material, or if it has serious privacy implications.

And that's just a proposed set of principles I came up with after three minutes of consideration—I'm sure with an extra 15 minutes we could come up with something even better. Point is, this isn't difficult. The fact that there are no bright clear lines doesn't preclude us as a society from drawing a line somewhere.