Becky had lived away from her family, had had to outwit those at her school
In the BBC series (not sure if it's in the novel - I'd have to check), when Becky is dismissed in disgrace from the school, she teaches all the girls how to say "Kiss my arse" in French (though doesn't tell them that that is what they are saying!).
I LOVED that moment - it was a real Becky-ism, whether it was accurate or not.
Becky was a survivor - she was quick, intelligent, selfish and almost utterly ruthless - because she'd had to be. Life had been hard for her, and she was determined to make it better. Even when Becky gets old and loses her looks, she will be able to take care of herself, because she's clever and pitiless.
Lydia Bennet was a spoiled brat - not terribly bright, but totally self-obsessed. Lydia spends all her time looking for someone who will confirm her opinion that she is The Only Important Person In The World - cute when you're 2, but people get sick of it as you grow up. She isn't cruel in the way Becky can be - but she is totally unfeeling and has no thought for anyone other than herself.
She will turn into her mother - a total social embarrassment, but when Wickham abandons her, or she gets sick of not having money because they have both squandered it, she will go off with any man who flatters her. When she gets old and loses her looks, she will have to rely on the generosity of her sisters - if they haven't abandoned her to her fate.
I can admire Becky, though I'm glad I've never met her, but could only find Lydia an irritation.