The fact that it was legally mandated is good evidence that it is [necessary].

P1 If X was legally mandated then that is good evidence that X is necessary.

P2 Slavery was legally mandated (in the USA)

C1 There is good evidence slavery is necessary.

P3 [insert other silly/abhorrent thing here: ICE in general, state murder...] was legally mandated.

C2 There is good evidence silly/abhorrent thing is necessary.

Personally I'd reject P1, the law isn't an arbitrator of mortality, or truth. Due to C2 I'd side eye anyone who says it is.