I would try having assassination being seen culturally as a sort of test or filter for rulers whose decisions are questioned (or their fitness to rule, etc.); if they cannot defend themselves from assassins, they have no business being rulers in the first place.
In any instance where a ruler is elected (party-based democracy works here, especially if the leader of a party is chosen by a vote amongst only higher-level party officials), it is unlikely that there will be a unanimous agreement. Some people will object, others will vote for the candidate because they are the least objectionable rather than because they like the candidate (see: the last U.S presidential election), still others to pay off (or gain) political favors, etc.
In a militaristic culture or even just one that values personal strength and cunning (possibly a society where "the strongest become king" or some such idea is a critical cultural point), assassination could be seen as a means of testing one's leader. All right, so they won the popular vote/won the support of high officials/inherited the position, but do they actually have the strength and intelligence to do the job? An official who has second thoughts about their support after the fact (see: Brexit, the last U.S presidential election) might quietly hire assassins to remove the ruler whether or not that official think they can take that place (so the assassins are not necessarily sent for personal gain alone).
This conception also works in a hereditary system, such as a monarchy; history is littered with examples of bad kings who only got the position because of bloodlines as well as good kings who genuinely did well. Assassination might be seen as an expedient and socially tolerated (if not exactly welcomed) means of dealing with the bad ones.
If the ruler can fight off the attempt, then they might be competent after all. Bonus points if cultural norms dictate that assassins are expected to use physical combat rather than tools like poison to eliminate the ruler and/or the rulers are expected to take a personal hand when necessary instead of relying entirely on bodyguards and the like. If the ruler dies, well, then they would have done badly in the position; the ruler's death in itself is a sign that they were not worthy of the position in the first place.