Considering the fact that the Catholic Church does not and did not recognize formal divorce, and I am sure that, statistically, at least one noble lord in Europe some time during the thousand years of the medieval period had his spouse killed, the latter.

Annulled marriages were not remarkable, but not common since they required episcopal (usually papal) authorization. The most disastrous of these politically was the annulment of Louis VII of France's marriage to Eleanor of Aquitaine.

Despite what I said above, I am actually unaware of any actual examples of a member of the nobility killing his wife because he could not get rid of her any other way. That doesn't mean it didn't happen - you can't prove a negative, anyway - but there it is.