Elijah assumed that based on Klaus's past history, he just happened to be wrong this time and that was most likely partly due to the fact that making hybrids that way hadn't occurred to Klaus.
Elijah merely stabbed Klaus to temporarily incapacitate him, he isn't going to allow either of his siblings to kill the other. And Klaus can only be killed with the white oak stake. Although I'm sure the writers will figure out some witch-ex-machina way to kill him.
You seem to forget that Klaus is the one who has daggered (incapacitated) and stored every one of his siblings (including the two that are now dead) for decades at a time, multiple times. And the only reason he hasn't lately (except Elijah, at the beginning of the season) is because the daggers were stolen.