The slayer rule is an incidental law resulting from a criminal conviction for murder, which states that a person cannot benefit from inheriting assets or property from a person whom they murdered.
In the slayer rule, the murderer would be skipped over as though they had already died, so whatever they stood to inherit would instead be given to their children or to the next in line.
The slayer rule was put into place so that a murderer would not be allowed to benefit financially from murdering someone that they would have inherited from. This would include any time that a spouse was killed by the other spouse or by their child.