Why would you... *SPOILERS*

Unless she wanted to be arrested?

My thoughts were that she picked it up knowing her fingerprints were already on it (from the earlier scene where Alex asks her to take out the knife and look at it and she replies "it's beautiful"), then she was gonna try to quickly wipe them off the murder weapon. Not sure but was my guess.

It's stupid ....

except Nina and Sara -- from the guy hitting on Alex at the bar -- was staging the whole thing. Sara allegedly went there to "warn" Nina -- but she didn't have to do that. Nina knew all about Alex & the fact she was a murderer. Alex calling the police on Alex also probably was staged with Sara's knowledge.

So touching the murder weapon would not be a problem -- the whole plan was to get arrested & turn Alex against Nina which eventually led to Alex's confession.