Why didn't Padme see Anakin slaughtering women and children as a major red flag?

There is an old saying:

"Love is Blnd".

If Padme already loves Anakin when he confesses, she may overlook that crime.

And there is another old saying:

"None is as blind as thouse who will not see".

If Padme is really in love with Anakin she may refuse to believe that he actually slaughtered women and children. She may imagine that he only dreamed that he did. Or that he wanted to kill the sandwomen and sandchildren and so his guilty conscience made him imagine that hte actually did so.

Or maybe Padme doesn't consider sandpeople to be people and thus thinks that slaughtering them is no big deal. I haven't the faintest idea what impression Padme from Naboo would have of the Tuskan Sandpeople of tatoooine. Do Tuskans live on other planet? Are Tuskans consdiered to be the scum of the galaxy? I note that the Naboo seem to look down on the Gungans on their own planet, and the xenophobic Palpatine came from Naboo, so possibly Padme comes from the same background and shares that attitude toward nonhumans.

And of course a women in an abusive relationship with a man will often "hide her head in the sand" and believe that he is not as cruel to her as he actually is. And as far as I know Anakin wasn't abusive to Padme but good to her, at least until their last meeting, so it would be much easier for Padme to ignore Anakin's cruelty when it was not directed at her.

Bad writing, this was one of the worst decisions of the prequels. There’s a reason The Clone Wars is considered the real Anakin story.

Maybe she thought it was a brain burp.

Because they don't see Tusken Raiders as people. Just like how you don't see a cow as a person. Plus they did just butcher his mother.

Yeah, and she finally tells Anakin she loves him half an hour later.

Which is one of those Lucas plot twists that's always made me wonder if Anakin was using Jedi Mind Powers on Padme to get her to love him, because that's the point where she should have gone from feeling dubious about him to feeling repelled.

She is still willing to forgive him when he says that he killed all the Jedi kids in Revenge of the Sith.

in the star wars universe men are men

As it has been touched on, considering the fact that it was sand people that Anakin killed who were long regarded as completely uncivilized, hostile aliens, it was probably easier for Padme to accept given the overall circumstances. I think it would have been VERY different had it been regular, civilized species.