Because voting is specifically a popularity contest, and most people aren't very politically engaged.

Blue is marginally better than red, in terms of trying to secure conditions where actual action can be effective. Summarizing another commenter, you're not voting for good vs evil, you're determining which mainstream option is going to be more difficult to fight against and trying to ensure they don't win.

I don't think you can erase the acknowledgement of "no child should be killed" from the masses. The people who already believe that aren't going to stop, and the ones who don't aren't going to start.

But we celebrate saving one child because, again, it's a popularity contest.

The red voters are pretty stalwart supporters. Their voting habits don't vary much no matter what their party does. They'll celebrate killing 3 children because their party tells them that they were the bad kind of children.

The blue voters are much more variable. They like to think of themselves as decent, principled, thinking people. When you inundate them with the child killing, they're more likely to stay home. And since blue is marginally better for us, that works against our purposes. Ironically, loudly condemning the killing of 2 children makes it more likely that it will be 3 children after the next election.

You're absolutely right that they're ghouls who don't deserve to be celebrated. But we don't celebrate them because they deserve it, we celebrate them to reduce the chances of getting the worse alternative.

Once the worse alternative is eliminated, and there are better alternatives that stand a chance to win by calling out the lesser evil for still being evil, we should absolutely 100% do that. But until then, I don't want to demoralize the voters who can help stave off the greater evil until we have a viable alternative, be it an actual leftist candidate with broad appeal or a sufficiently organized revolutionary force.