I liked it overall but the facial expressions were the most striking.
It took me awhile to see that there are four aliens in this.

i like the character design and the expressions are very appropriate.
the background is well done, but i think it lacks the alien quality that the figures have. it looks like they are in a national park in north america.

Wow, great artwork here.
I can totally picture an "insert funny caption here" type situation for this.
Although inter-species murder is never funny... sometimes!
XD

criminal alien o.o

It's actually a pretty cool image, I like it. But I think your colors blend a bit too much, specifically the alien on the right and the rock in the backgrond.