I would argue that any act, magical or not, has an effect on your 'soul', but the vast majority of acts are negligible in game terms. Killing people without the use of magic still makes you a bad person, but using magic is using a part of who you are to do it, thus magnifying the effect it has. I'd say that using summer magic to kill would turn you more summer-y and winter magic more winter-y, both of which would make you less human, more monstrous and so more likely to do it again. Hellfire would obviously leave a mark on your soul and soulfire is your pure soul itself used as fuel.
I think the point of the first four laws is to make sure wizards aren't turning in to monsters (losing refresh level in game terms), especially as monsters that are just following their evil nature are more likely to break the last three laws, the ones that upset the natural balance and open the doors for the outsiders.