Bowser’s main claim to fame is that he has an unhealthy obsession with Peach, who he tries to kidnap every once in a while. His main goal is usually to force Peach into marrying him, with annoying Mario simply being a fun side effect of his modus operandi. While he
has never succeeded in marrying Peach (except this one time by accident), there’s nothing to indicate that he’s going to stop trying anytime soon. Nintendo tried to make a running joke of his kidnapping ways, but when you stop to think about it, it’s a messed-up personality quirk to have for someone who they have tried to turn into an antihero in recent years.
Among other despicable things he has done, we also can include that time he convinced his son that Peach was his mother, thus letting his kid do all the dirty work for him.