Here are some options, with explanations of their connotations:
* 野郎 (yarō): This is a very rough and informal term for "guy" or "fellow," often used in anger or contempt. It's closer to "you bastard!" than just "bastard." It's extremely vulgar and should only be used in very informal settings, if at all.
* 畜生 (chikushō): This means "beast" or "brute" and is used to express extreme anger and disgust towards someone. It's much stronger than "bastard" and implies someone is cruel or inhuman.
* 汚い奴 (kitanai yatsu): This translates to "dirty guy" or "filthy fellow." It implies a morally reprehensible person.
* 最低なやつ (saiteina yatsu): This means "the worst guy" or "a terrible person." This is a less intense option than others, though still negative.
* 私生児 (shisei-ji): This literally means "illegitimate child," and is the only direct translation for the "bastard" meaning of an illegitimate child. However, it's very formal and rarely used in casual conversation.
To choose the right word, consider:
* Are you referring to someone's illegitimate birth? If so, use 私生児 (shisei-ji).
* Are you expressing anger and contempt? 畜生 (chikushō) or 野郎 (yarō) might be appropriate, but be *extremely* careful with these.
* Are you describing someone's character negatively? 汚い奴 (kitanai yatsu) or 最低なやつ (saiteina yatsu) are better options.
It's highly recommended to avoid using any of the harsher terms unless you are absolutely certain of the context and your relationship with the person you're addressing. Misusing these words can be deeply offensive.