Chastise

Removal that only fires when the creature is already swinging is a trade Wizards has used to price down white's answers, and this is the lifegain-flavored version of that bargain. The restriction to attacking creatures means you cannot use it proactively to break up a board or pick off a blocker; you wait, take the declaration, and then erase the threat at instant speed before damage resolves, banking power's worth of life on the way out. That timing window is the whole transaction. Against an aggressive draw it reads as a one-card swing: the attacker dies, the race tilts back, and the life total moves in the wrong direction for the attacker twice over. Against control or a board that sits back, the spell may never find a legal target. The conditional is what pays for four mana buying both unconditional destruction (no regeneration clause, no "nonblack" hedge) and a life swing scaled to the creature's size, so the bigger the thing coming at you, the better the deal. It is defensive in the most literal sense, built for the white deck that wants to survive the early turns and grind, and it asks you to play reactively rather than dictate the board. Plenty of white removal since has carried looser conditions; Chastise belongs to the older school where the spell rewarded you for letting the opponent commit first.