I don't think it was Samuels intention to hurt Kiwi on that play but I can sorta see why he tackled him after he got beat around the edge.
It's just one of those things where you get caught up in the moment and try to do something quick or else your qb's gonna get smacked.
Samuels did seem concerned right after he did it and approached Mathias right away to see if he was ok.

What's your thoughts?

I think it has to be considered a "dirty play"

The Redskins were doing a last ditch long pass to try and win the game

Samuels was beat by Kiwanuka and Samuels tackled him to stop him getting to the QB

That is a very blantant professional foul that was completely outside of what is acceptable for an offensive lineman to do.

It isn't like it is a bit of a hold; it was an around the legs tackle and would probably have ended in either a sack or a disrupted play.