Does having unwanted or blasphemous thoughts mean I'm a bad Christian?

No. Having a thought is not the same as believing it, wanting it, or acting on it.

Everyone's mind produces thoughts they didn't choose and don't agree with—including thoughts that shock the very person thinking them. If your faith matters deeply to you, your mind may throw exactly the thoughts that scare you most about it. That's how intrusive thoughts work, not a special punishment for Christians.

A thought that horrifies you, that you immediately want to push away, is very different from a belief you've settled into. The horror itself is usually a sign your conscience is working, not evidence something is wrong with your faith.

The Bible treats temptation and sin as different things. Being tempted by a thought isn't sin; choosing to act on it or dwell on it is.

If these thoughts are frequent and distressing, that's common and can be helped. It doesn't make you a bad Christian—it makes you someone worth being gentle with.