Alright kiddos, I gotta just say this thing. BAD PEOPLE =/= BAD CHARACTERS.
Severus Snape is motivated by the love of a woman who never reciprocated his feelings. He latched on to her fleeting affection, and used it to justify being horrible to her child and his friends (and just children in general.) He joined an Order that hurt and killed other people, and only questioned it when their work threatened Lily. Not her husband, or her child. Just her. His betrayal was as selfish as anything else he ever did, and his redeeming moment came when he questioned Dumbledore’s method of raising Harry Potter to die. It was not enough to make up for a lifetime of selfish, abusive, dangerous, habits. Severus Snape was not a good man. He is however a fascinating character.
Kylo Ren is someone we don’t know a lot about yet, but he can be summed up as a man who is doing terrible things in the name of what he thinks is right. From what I gleaned from The Force Awakens he is attempting to complete his grandfather’s work, and fulfill the prophecy in the way he sees it. Which is very fucked up, and involves killing lots and lots of people. Also involves invading people’s minds violently via the Force to steal information and otherwise torture them. He’s made interesting through their use of his youthfulness, the erratic temper inherited from his grandfather (and probably the Dark side,) and his attempt to train Rey. He is a very bad man, but a very fascinating villain.
I’m not advocating to romanticize either man. Snape is that ‘nice guy’ who breaks when a woman says no and at best spends the rest of his life creeping around the corners of hers and telling people that he deserved her and how dare she choose someone else; at worse, this is the type of man who goes on to kill that woman, or others, because of his inability to accept rejection and move on. Kylo Ren attempted to forcibly invade Rey’s mind. He is abusive, and dangerous, and willing to kill her and her friends in order to achieve his goals. This is a man who has already killed his own father at the whim of his master. Shipping him with Rey is gross, and violating, for a number of reasons.
That being said, I also don’t advocate for dismissing them out of hand. Their presence - as deeply flawed, uncomfortable, men - gives our protagonists something to struggle against and overcome. Their flaws highlight the paths our protagonists choose not to take.