Answers to Questions by Yair Davidiy (15 August 2017, 23 Av, 5777)
What are the worst sins a Jew can commit according to Judaism?
He will be Killed BUT shall not Transgress!
The Three commandments for which a Jewish person is obligated to die if necessary and not transgress are:
Prohibition of Idolatry.
Do not worship other gods. Missionaries who try to convert Jews to Christianity may not realize how much HARM they are causing.
Prohibition of bloodshed.
Do not kill another person unless in self-defence or something along those lines.
This means homosexuality, incest, adultery, etc.
To some degree a non-Jews is also obligated by these commandments but not necessarily bound to give their lives for them.
On the other hand the application of these commandments to a non-Jew in some cases may be stringent.
For details go to the relevant Hebrew language article and Translate it: