>humans only die to monsters if they're suicidal
You play as the absolute antithesis of a quitter, AKA someone so determined that time rewinds if you die so that you can try again, and everyone has died at least a few times on their first few runs of the game. Toriel can kill you without meaning to, due to just wanting to scare you off. Asgore clearly takes no pleasure in killing you and REALLY doesn't want to, clearly shown by him not being able to bring himself to look you in the eyes as he does it. Other common enemies that don't even realize you're a human have some pretty difficult attacks that can easily kill players on their first run. IIRC I'm not so sure that humans can only die to monsters if they're either suicidal or completely retarded.
>he murdered kids in Undertale and was a coward
I look at what Asgore did in Undertale, and I can't imagine myself doing it much differently other than watching Alphys a lot more closely. He deals with the depression of having his adopted androgynous child commit suicide, then have his son get slaughtered and barely make it back home just in time to die in front of his parents, for one thing. That's sure to bring on some immobilizing depression in anyone, regardless of leadership status. That, and he has an excuse of not wanting to start another war. Killing a human, taking their soul, the going outside and killing more humans to break the barrier is bound to cause some really nasty shit to happen that he wants to avoid.
He wanted to keep his people sated by having any human that happened to fall down be harvested for their soul as a way to buy as much time as possible while Gaster and Alphys tried to find a peaceful alternative, which leads into the next point. Before each experiment, there was no precedent for what they were trying to do. Both Gaster and Alphys were basically pioneers in the field of science in monster society, and nobody knew what the fuck their experiments would even cause.
IIRC he (and potentially his followers/the goners) was/were the only victim(s) that only resulted in his/their existence from being erased from everyone's memory. Asgore would only be aware that he had a missing royal scientist and not know why. Alphys was his next best bet.
Alphys is shown to be very secretive about the results of her determination experiments, and still kept trying even after the first few fuckups permanently scarred and deformed multiple monsters. The blame for the amalgamates is purely on Alphys. I'm still mad that she didn't get any character development out of that other than getting a tomboy GF ||with barely any believable chemistry. Fuck you Toby.||
Did Asgore fail? Yes, he failed. The only reason anyone got a happy ending at the end of the True Pacifist route was because of the player being a living Deus Ex Machina. However, every single possible odd was against him, and all he felt he could do was either wait, or start another genocidal war he wanted no part of.