I will agree with one thing. Both weapon attacks (main hand and off hand), should occur at the same time, thus if it's enough damage to kill after both attacks, then stealth isn't broken. Even in pnp, you make both attacks as an action and bonus action in the same turn. So in this case, if it kills, then the other enemies won't be alerted.
The rogue would be far superior for this, since you get advantage on the attack and sneak attack damage, as well as advantage on with the off hand due to surprise attack. The warlock would get advantage, but no bonus damage.
