Answer: Zookeeper Rick here, thanks for your question! It is an excellent one. It may sound strange and counter-intuitive, but it is indeed true. The method behind the madness is likely that if a mother kangaroo gets killed and eaten it can no longer reproduce, so in a sense the mother is sacrificing one baby so that many more can live.
That is actually not the only reason a mother kangaroo will sacrifice its baby, though. When a mother kangaroo has multiple different babies in various stages of development (for example one still in the fetal stage, one in the pouch, and one being weaned from the pouch), during harsh periods when resources are low a busy mom may not be able to support all three babies, and so may sacrifice one to boost the others' chances of survival.
As humans this behavior may sound appalling to us, but we rely on much more than rudimentary instincts to survive. Sacrificing babies for one reason or another is actually relatively common throughout the animal kingdom, for example some spiders such as black widows will eat their offspring as they hatch, and some monkeys will drop their offspring from trees if they are deemed unfit to cope with their group's social structure. In this sin-infected world where death threatens every living creature every day, God gave humans much more than just instincts to preserve ourselves and our kin. He gave us his own breath (Genesis 2:7) and created us in his image and likeness (Genesis 1:26-27) so that we are all God's children and equally have a purpose to exist regardless of the circumstance. We are not expendable kangaroo joeys or one-in-a-million spider hatchlings, we are indispensable, one-in-one unique and loved human beings.