I've recently been DMing a completely novice party through the 5e starter set - The Lost Mines of Phandelver. I'm the most experienced tabletop player out of the group, but that's not saying much as I previously hadn't touched a d20 in around 5 years. It's been a lot of fun so far.
Anyway, last night a somewhat sadistic member of the group asked if he could attack to maim a Redbrand Ruffian (in this case, a human warrior). Specifically the player wanted to hack off his leg at the knee. The particular enemy was at 1hp and grappled by another player at the time, so I suspected that physically it should be possible. Hence I described the 'killing' blow as having severed the leg, the pain of which left the enemy unconscious and bleeding out.
One of the other players, however, was unhappy with this brutality and decided they wanted to heal the Redbrand using cure wounds. They specifically asked if it was possible to try and heal the leg. I ended up ruling that it was, under the justification that it was a fresh injury. I made up the house rule that if the leg had been left to heal as an amputated stump, cure wounds would only be able to restore the amputee back to that poorly healed point.
I'm worried that I may have unknowingly opened a can of worms with this ruling, and am wondering if there is any official guidance on this? Additionally, I am wondering if enemies should be similarly permitted to 'attack to maim' the PCs?