The issue with speed loaders is, most modern revolvers reload by rotating the cylinder (the revolving component that holds the rounds) out 90 degrees. This is fine if you have two hands, and are right handed. A speedloader will let you quickly swap in a fresh load of shells.
The problem is, if you shoot left handed, or you don’t have a second hand, revolvers can be a real pain to reload. You need a place to actually brace the revolver safely, while reloading. If the character only lost their right hand, then they can brace the revolver between their torso and arm, and use a speedloader or just load it one round at a time.
But, if they’re left hand is missing, the revolver will open into their torso, if they try the same thing. They could flip the gun over, but that would be awkward as hell, and we’re still at the point of needing the forearm.
I believe you can modify some revolvers to load left handed, but I’ve honestly never really looked into it. I just rotate the revolver 90 degrees to get access to the cylinder with my right hand.
Now, there are revolvers that breach open, but, these fell out of favor for one reason or another. Those could be reloaded in the armpit, using a speedloader or a fresh cylinder. But it’s still a less than ideal situation.
Finally, there are revolvers that can’t use speed loaders at all. I’m thinking of the Peacemaker here, and it’s modern replicas. It’s a weapon that can be fired very easily and comfortably one handed, but reloading it does require two hands, one round at a time. (Actually the Peacemaker is more comfortable when held one handed rather than in a modern Weaver stance.)
Either way, a character can shoot someone else if they only have on hand, and that was the point I was making.