"Most spells are a combination of an intent and a focusing gesture and incantation. The focusing gestures and incantations are more developed than discovered but spell development's dangerous because minor variants of even very benign charms can do bizarre and occasionally fatal stuff. The intent for most spells can be partial - you can cast 'Lumos' knowing only 'it does something related to light' - but not incorrect - you can't cast Lumos believing it sends a blazing ball of light flying at your enemy. It's pretty rare for spells to require harmful intent - most lethal spells, for example, require that you intend to kill someone but not that you intend to in so doing commit a wrong against them - but that's just because spells that do simple things are much more common than spells that do complicated things, and spells that have simple attached intents are much more common than spells with complicated ones.
More powerful stuff usually has more complicated attached intent. Though, honestly, it could be that there're less powerful spells that have complicated attached intent and no one's stumbled on them, if there were a flower-growing spell that required you to passionately desire the death of your spouse it just never would have caught on. It's also harder - and more dangerous - to cast with partial information. Usually won't work at all, if it does work might have some kind of unexpected backlash... it is totally plausible you couldn't cast the Killing Curse on someone if you expected this would cause them to daevafy, and it might depend on whether you conceptualize that as death. You get better at spells with practice, and that's partially getting better at the focusing and it's also partially getting acclimated to the mental state for casting it from, so that it's easier to slip back into. The other two Unforgivables, inadequate intent just results in the spell failing, inadequate experience or power results in the spell not being very powerful - snaps without direct attention, in the case of the Imperius, and is easier to break, and requires more micromanagement while lending less fine control. I think the Cruciatus is just less painful if you're not very good at it. Avada Kadavra's all or nothing so I don't think there's a getting-better-at-it, just knowing how it's done and being in the right state of mind."