Why does Nick have to shoot and kill the Wesen that are doing wrong things? Can't he just arrest them and give them to the Wesen Council to judge them or something?
From what little we've seen, the Grimms aren't exactly keen or working with the Wesen Council, or vice-versa. Nick in particular has a very antagonistic relationship with them. So far, we've seen a number of indications that the two groups want nothing to do with each other:
- Monroe was warned not to even tell Nick about the existence of the Wesen Council.
- The Council has its own operatives, which it was willing to send to work against Nick in a few cases.
- We know the Grimms, Royals, and Council used to have an alliance, but that no longer appears to be true.
- What we know about the Council seems to imply they're pretty quick to kill as well, even more quickly than Nick is.
Overall, the relationship between the Grimms and the Council is barely "cordial"; there seems little change Nick would actually cooperate with them unless something changed dramatically.
He could be usually most of the Wesen that Nick kills are dangerous and tend not to go down easily. So it's more of a kill or be killed situation with the Wesen he kills. There is also the problem of storage of the Wesen until the council can come pick them up and do whatever it is that they decide to do with them. Keep in mind that Nick usually reasons with those that can be reasoned with and they either leave if they had a good reason to do what they did or Nick dispenses "justice" to them since no human court of law exists to deal with such matters.