Board Thread:Questions and Answers/@comment-33035590-20170905095350/@comment-13682699-20170905125409

He doesn't necessarily have to 'kill' them per se. However doing so without resorting to violence in general would require him to either essentially steal/commit grave robbery and take unwilling SOULs of deceased from families (which would go just as well as him just slaughtering innocent people) -OR- he would have to be patient and take his time.

By that last statement, I mean he would have to come out at the most opportune time when humans would not still be freaked out by another large and intimidating goat-like monster wielding a trident who came from underground desiring to take a few SOULs to assist him in his endeavors to release his fellow comrades from underground (Sounds a lot like the devil trying to collect SOULs to unleash his army). Once that blows over (which could take a LONG time) he would still have to find 6 WILLING volunteers to donate their SOULs in order to break the barrier (which could also take a LONG time).

So ultimately, there is no solution where breaking the barrier shortly after the death of Asriel and Chara would not result in catastrophic failure. If he collects them to soon through violence or deceit, he is going to start a war whether he wants to or not and this time either monsters or humans will be COMPLETELY eradicated(worst-case scenario). If he tries to communicate to humans peacefully so shortly after the incedent with his children, he would have died like his son before him and the Underground would have no capable ruler -Toriel at that point would be overthrown if she tries to instate a peaceful policy for any further fallen humans, which she WILL try to do- and would have be back to square one as far as possessing human SOULs and possibly fall into despair and dying out before another human falls, never even breaking the barrier(worst-case scenario for monsters).

Because of the reasons stated above, we can only conclude that Toriel was just trying to jab at Asgore even more when she was raging at him by giving him any plan that involved him actively searching for a solution to the barrier instead of waiting for humans to fall and giving false hope (which wasn't so false since 7 humans EVENTUALLY fell). We know at this point that no plan that Toriel could come up with would solve the issue and allow the monsters to leave the underground anytime soon.