If a Slayer is killed by normal Close Combat Attacks, it will immediately make a single Attack back against the unit or model that killed it before the Slayer is removed as a casualty.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
If a Slayer is killed by normal Close Combat Attacks, it will immediately make a single Attack back against the unit or model that killed it before the Slayer is removed as a casualty.