Letting a zombie grab at someone just for the hell of it, even if he knows the window probably won't break, isn't exactly the friendliest behavior. That's not even getting into Nate "covering" Russell. And that slasher smile he put on after he asked if they were going to kill the old couple and take their stuff... that is not a smile that convinces me Nate's mental faculties are operating at peak condition, if you catch my drift.
By which I mean, he's fucking insane.
Well you could count the "covering" thing as testing him. If Russ couldn't get through one little track of bullets, kid wasn't worth keeping around anyways. Little morbid, and definitely out there, but not fucking insane for sure. Same could be said about the zombie. If Russ didn't have the strength to keep his arm away from one zombie, then the kid was going to get bit in minutes.
Although I agree that Nate was most likely "testing" Russell, he's still fucking insane.
...In all seriousness, Nate was just a jerk. Testing him was smart, trying to see Russel's "worth". It also seemed that Nate was simply using him.
"You're my bitch now..."
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She'll definitely be in Season 2, but I don't think she'll be extremely good for Clementine. She seems sort of irresponsible or more insecure. She doesn't seem like a very brave person. Then again I don't think any of the 400 Days group are that brave aside from Vince and probably Tavia considering how she approaches a group while outnumbered.
Though courage isn't everything, and it can be assumed that Vince is a good shot, while Wyatt is the best in the group (considering he can shoot about five walkers in the head, in mist, while dragging a dying man with him.)