If the warlock uses Armor of Agathys that's great, but they are right next to creatures, wherein lies my issue. It would have been likely, but not absolute, that those minons would have gotten in an attack.
Actually, the minions take damage
before they get to make an attack -- they take damage as soon as their turn rolls around, before it takes any actions.
The only way for a minion to get in an attack is to run up next to the Warlock deliberately... and that's suicide.
If you feel minions are being denied their rights to an attack, consider the difference over a few rounds, from the perspective of three minions:
Case A (auto-kill like Armor of Agathys, Rain of Steel, or Flaming Sphere)
Round 1: The 3 minions either win initiative, or don't. If they do, they get one attack each. If they don't, the PC moves his effect (which may require moving himself) right up next to them, and they all die this round.
Round 2: Minions are already dead, or they all die now.
Attacks: 0 or 3; expected number of attacks 1.5, subject to expected initiative differences.
Case B (auto-kill via Rod of Reaving)
Round 1: Warlock auto-kills one minion; depending on differences in initiative, 2 or 3 attack him this round.
Round 2: Warlock auto-kills second minion; 1 or 2 attacks this round.
Round 3: Warlock auto-kills last minion; 0 or 1 attacks this round.
Attacks: 3 or 6; expected number of attacks 4.5, depending on initiatives.
- - -
Clearly, auto-killing one at a time is much "fairer" than auto-killing in huge batches. The number of attacks any group of minions gets in case A is
linear (one attack per minion), while the number of attacks the group gets in case B is geometric (the minion population charted over time is triangular).
Cheers, -- N