Is there anything more subtle or less obvious? I don't mind if it's equivocal.
closed as too broad by Jason Bassford, ColleenV♦ Jan 21 at 12:13
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
In general, actually using any onomatopoeia to indicate the evil laugh will be particularly blunt and unsubtle. There are many such onomatopoeias, as Robert W linked here. None of these are subtle. The subtle way to indicate this kind of interjection is to use description and prose.
As such, it is better to simply state that the person is making an evil laugh outside of the quotation marks, as follows:
"And then," he said, adding an evil laugh, "I will shoot him in the head!"
When using a script format, such as for a screenplay or theater production, you wouldn't write out the evil laugh as "mwahaha", you would instead include a stage direction for the actor to come up with an appropriate evil laugh. Homer Simpson's famous "d'oh!" line is listed in the script as "annoyed grunt" after all.
In reflection of this, when writing informally on the internet, or a chat room, or on social media, simply writing
would be the appropriate way to go. Where italics are not available on your platform, surround this text with asterisks (*) or use the word "insert", perhaps with a parenthetical, as in:
I will destroy you!!!! (insert evil laugh here)
I'd add "Ahahahaha!". Used particularly well in Thriller. Also "Heh heh heh...".
The effectiveness of onomatopoeia for evil laughs is largely dependent on the delivery. Actors in evil roles can do these kinds of laughs in movies, but in everyday conversation, even a high-quality attempt would garner (ridiculing) laughter from anyone overhearing it.