He had more pressing matters, like removing her doomsday device and fixing the damage she did to one of his loved ones.>>112039879>>112039910
Let me take a few minutes of your time to explain why I think your way of thinking is flawed.
You say stuff like that while making no attempt to understand what was going through the head of Steven. He just saw this woman attack his family, put them in a short comma, then when they came to they didn't remember anything. He did what most people would, he tended to his loved ones first. You also fail to take into account the fact that Steven was terrified of the woman, he even says "She looks nice now, but if she regains her memories she'll kill us all"
Despite all this during the garden scene after hearing what happened to her, something that I'll argue most people possibly including you guys wouldn't have given a damn about considering the situation, he spends the time to try and give her what he thinks she needs. He tries to give her hope that the future will get better, and only after he thinks it sticks does he go on to then bring her out of the garden to remove the thing that's going to kill everyone. The only thing he did wrong was that in his haste to get to an ending where everyone is safe and happy is that he failed to truly see that Spinel needed more than an assurance in a future, she needed assurance that she wouldn't be abandoned again.
But I can't stress this enough he only asked Spinel to remove the injector after he thought he successfully helped her, and he did just that ask. He didn't demand she does it, he didn't (Purposefully) hold his friendship over her head like a carrot on a stick. Even when the time for the earth was getting close to running out, he still tries to keep her feelings in mind. And the fact that he thought he helped her get what she needed is why he is confused as to why she relapses because in his mind she was fine it was Garnet's turn to get help as she wasn't.