And that is exactly what ICE is trying to accomplish as stated in the OP.

But they're trying to accomplish things other than that. They're also trying to get rid of lots of people who are not rapists and murderers. And the Democrats and leftists are objecting to ICE trying to get rid of all those other people. Not to them getting rid of the rapists and murderers.

This is pretty basic stuff. If a policy will have two effects, A and B, someone can object to the policy because they disagree with causing A without necessarily supporting B - or support the policy because it causes A without necessarily wanting B.

The classic example being our requirement that accused criminals be given a trial by jury and only convicted upon proof beyond a reasonable doubt. That has the effect of reducing the number of innocent people in jail (good!) and increasing the number of guilty people who are not convicted (bad!). But someone can support due process because they want to protect innocent people without it being true that they want guilty people to go free.