This is slightly misleading. While it is true that the person with the highest probability of murdering you is your partner (at least, if you're a woman), it is more likely that the murder is a stranger to the victim than to be the spouse of the victim.

For example, in the FBI's 2011 dataset [1], of the 7076 murders by someone with a known relationship to the victim, 1295 were by a husband, wife, boyfriend, or girlfriend. This is compared to, of course, the 2700 murders committed by acquaintances and 1481 by strangers. From this, we can see ~18% of murders are by their partners and ~60% are by someone quite a bit further flung.

The reason why both statistics are simultaneously true is that most people have only a few partners (focusing the entire risk on a small group) where as there are many thousands of people in the latter categories (creating a very diffuse risk).

So, if you were to bet one which person murdered someone, you would do well to guess one of their partners. If you had to guess if the victim knew their murderer, you should bet that they did not.

[1] https://ucr.fbi.gov/crime-in-the-u.s/2011/crime-in-the-u.s.-...