I was a little late to this fanwagon, but I seem to have caught up to it and jumped on.

As is customary, spoiler content ahead, under the fold.

One of the obvious themes of Undertale is that it's always better to do the right thing, and you're fully responsible for every instance in which you choose to not do the right thing. The subtler second point is this:

If you do decide to do something horrible, flaking out and doing it halfway will be worse for you than if you go through with it properly.

We see this happen multiple times:

• Asriel agrees to help Chara commit suicide and murder six humans to break the Barrier, but backs out halfway through the plan, and ends up with Chara dead and himself stuck in a flower.
• Asgore declares war on humanity to escape the Underground, but sits and waits for seven humans to wander in rather than go out and fill the soul quota himself after the first, which leaves all the monsters just waiting around.
• Most of the game's most interesting content is concentrated in the Pacifist and Genocide routes, so killing some people, but not everyone, leads to relatively boring endings.
• The sign at the beginning of the Ruins hints at this last point for the player: "Only the fearless may proceed. Brave ones, foolish ones. Both walk not the middle road." It doesn't matter whether you consider fighting "brave" and pacifism "foolish", or mercy "brave" and violence "foolish"--the point is, being afraid to be either ideal is not the way to go.
• At the end of a Genocide run, Chara only scares and mocks you if you try to back out at the last second and not finish ending the world...and you only lose your SOUL to them if you persist in undoing what you've done.

Note that cutting one's disastrous actions short usually results in things being better for "the world" or other nearby people: it's the perpetrator themselves that gets extra-punished for being only halfway bad. A complete sociopath doesn't suffer from causing others harm. A person with empathy, who feels guilty after harming others, has the capacity to stop themselves and also to feel bad about what they've done. A tyrannical leader can at least be admired on some level for getting things done--getting anything done--but an ineffective, indecisive throne-warmer is despised by all.

Poor Asriel. He suffers the most out of anyone in this game, and his only crime was that he let himself get talked into a plan that he knew was bad, and later decided he couldn't finish. But that's a pretty big deal. If following the bad road only halfway is worse than going to its end, it leads back to the main point again: do not take even that first step.