A word that defines when you have to consider multiple factors when making decisions

You might want to use 'prepotent':

Having great power, force, influence, or authority; pre-eminent in power; more powerful than others; predominant.

[From "prepotent, adj.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/150513 (accessed October 23, 2015).]

Possibly you could use the rare 'magnipotent':

Possessing great power.

[From "magnipotent, adj.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/112347 (accessed October 23, 2015).]

There is also the obsolete and rare adjective 'maximious':

Having great power.

[From "† maximious, adj.". OED Online. September 2015. Oxford University Press. http://www.oed.com/view/Entry/115267 (accessed October 23, 2015).]

If it's deliberate - antagonistic (link)

1. acting in opposition; opposing, especially mutually. 2. hostile; unfriendly.

or troublesome (link)

1. causing trouble, annoyance, or difficulty; vexatious

or more awkwardly - troublemaking

If it's careless/reckless, maybe boorish (link)

1. of or like a boor; unmannered; crude; insensitive.

or loutish (link)

1. like or characteristic of a lout; awkward; clumsy; boorish.

Edit: Leaving my original response as is, but to address the elaboration on the original question:

I think the question of motive is really important in this scenario. In your parking lot example, the person parking outside the lines has done something wrong. But I can see plenty of motivations for the person parking up close to their driver-side door, and each infers a range of different adjectives.

• They're doing it to deliberately inconvenience a wrongdoer, or demonstrate the wrongdoing to the perpetrator. (antagonistic, indignant, vindictive)
• They're doing it because the parking spot next to the wrongdoer is superior in some way to an empty group of spaces, and they refuse to modify their behaviour just because someone else has done the wrong thing. (unyielding, obstinate, stubborn, uncompromising, adamant)
• They're doing it in the hope that it will start a confrontation with the wrongdoer. (confrontational, pugnatious, belligerent, bellicose, combative, adversarial)
• They're doing it so they're not inconveniencing a third-party by parking outside their own lines. (righteous, virtuous, principled)

I think antagonistic is still probably the best general answer, but hopefully some consideration of the different motives might narrow it down to something more accurate.