Assasin snails are only called so because of there colouring and stripes - they were just doing there job by tidying up! Snails can however over breed so it is worth keeping there numbers down if you can.
It would have been dead or close enough before they started eating it- an assassin snail wouldn't be able to catch a tetra unless it was on the verge of death and wasn't moving. I'm sure the rest of the fish will be safe from the assassins. How big is your tank? What are the water readings?
Oh My God... we got 2 assassin snails about 4 months ago, we now have about 10. We kept them all to sort out the problem snails, came into front room this morning and there's 6 assassin snails eating a tetra! So what do i do now? Should i get rid of some assassins? I'm guessing the tetra was maybe poorly? He seemed fine though, colourful and perky, swimming and eating fine, no unusual marks or splodges.. Sooooo weird.