How should European countries react on the NSA activities?

[Husar]

He rigs elections

Yes, that is a vice but also not proven.
Given the current state of our media (let's just say "Captain Sum Ting Wong" here) these allegations might as well be completely false.
According to our media the muslim brotherhood and the syrian rebels were also fighting the same fight as the french revolutionaries.

has his opponents arrested

Yes, but Snowden is wanted and quite a few opponents of the US president are currently in gitmo.

charged with blatant lies and convicted by school teachers tanding in as judges

According to our trustworthy news organizations and their thorough, unbiased research.
Apart from that, at least they do get a trial unlike the people in gitmo.

he has Russian Citizens in other countries assassinated in ways that make it obvious who orchestrated the assassination.

Like I said, at least he is relatively honest and direct, these people knew he was out to get them and why.
Unlike the USA, which abducts and kills people all over the world, sometimes accepting to blow up a whole wedding party as well.
When Putin kills his opponents he does at least not cause great "collateral damage" that he explains away as "yeah, but I'm the good guy anyway..."

The US is a bad ally - Russia is an ideaological enemy we happen not to be at war with.

Given what I just described, the USA seems more like an ideological enemy than Russia. How you can equate Putin with Russia after saying he rigs elections is also quite strange. If he represents the Russian population as a whole then the elections can't have been rigged.

I can only recommend you take a more objective look instead of ignoring all the recent human rights abuses of the USA just because they were traditionally better and historically on our side.