I haven't been able to replicate the bug but I think I can see a cause for this.
Basically the ranged attack has no cooldown at all which in theory shouldn't be a problem. Normally doing an action should block it from doing any extra attacks. Something must occasionally go wrong in the attack distance/speed calculation making it think it finishes much faster than normal. Not entirely sure what happens but the result is probably rapid fire attacks.
The best solution seems to be to add a cooldown which should hopefully eliminate the possibility of spamming.
Massacre works much the same so unfortunately the buff doesn't apply to the stats just to the direct damage as it happens.
Just testing the Massacre stuff revealed that it wasn't quite doing as it described, the numbers output after adding the talent didn't make sense.
After the correction, simple test Assassin using Melee Attack in full medium gear:
No Massacre: 402 damage
1 point Massacre: 450 damage
2 points Massacre: 498 damage
It not affecting DoTs is how the system works. DoTs are generally of a damage type that is not melee/ranged/spell even when those values are used to calculate the damage.