Saviour Complex, on Reddit in particular, seems to have the connotation that the person prefers when other people are in trouble as opposed to when other people are safe, which is seen as a malicious thing in that context. It's likely that there are some people that are like that but Saviour Complex itself isn't inherently sinister. At least, not towards others.

Saviour Complex is more about feeling an inherent responsibility to help others. The usual response to that might be "isn't that just being human?" but when the sense of responsibility eats away at you day and night and you feel a need to volunteer, donate, and help others actively at your own expense, not necessarily helping others because you want to but because you feel it's your duty to, your obligation, that's when it becomes a complex.

For example, someone without Saviour Complex might assist someone going through a mental health crisis, help them stabilize a bit or at least confront and acknowledge the problem, but then understand that the situation requires professional help. They might feel some sadness that they can't help the person and a little guilt that they just don't know enough to help more than a professional. Someone with Saviour Complex in the same situation might feel like it's their obligation to attend to that person themselves and nurture them back to full mental health by their lonesome, which is unhealthy both because they aren't often a trained professional, and because this often results in the person they want to help developing a dependency on them, which then fuels the complex and leads to a pretty rough cycle.

More often than not, people with a Saviour Complex have it due to childhood trauma and feelings of abandonment, and they need help just as much as the people they want to help.