Alma 34:12

But the law requireth the life of him who hath murdered; therefore there can nothing, which is short of an Infinite atonement, which will suffice for the sins of the world;

But the law says the killer must give his life. So only an endless payment will be enough for the sins of the world.

The law requires the life of the one who murdered. So nothing less than an infinite atonement can pay for the sins of the world.