Also, I want to point out that we don’t know if this is the same kingdom or even how long has really passed since her first life ended.
She could have also got reincarnated in another world, thus no connection at all to her previous life.
I think her new life is also bad for her development. Girls from nobility seems to not leave their family house until their social debut. They only interact with family, servants, and private tutor who make sure they're presentable. So there are not much variety of personality they can know while growing up. She's also an only child and her mother doesn't like socializing (means she also got no close friend she can invite for casual private tea party).
Add that to the preconception she has about how human are hypocrite.
For her training, she only started to seriously consider it after the dog incident, seeing how weak the security they have.
Assassins guild is just a familiar place she know can provide her with real world training. Also, in her worst case scenario where she must lost or abandon her family, the only place Serena thought she the only skill she had to work in is assassination business. She probably thought it's better to join early rather than beg them in desperate time.