Of course sinister thoughts are sin, that's pretty much the nature of sin. Regardless of whether you act on it or not, that thought was still in your head. And of course you're doomed. Every human being is doomed, why do you think there is religion? Humans want to know that they're not going to just live on this earth and then die. I don't want to say which religion is right, because I could go on and on, but if you want to talk, feel free to send me a message.

it depends what you regard as sin, your moral compass if you like. personally id say sin is irrelevant because i think when you die thats it caput, finito, goodbye and goodluck, adieu. However, if your convinced death and everlasting life have something in common id be worried because everything you do could be a sin it just depends on the higher power that is judging you. My only comment (bit paradoxical that) is that you should live and act and say things that you consider to be "good" i stress "you" not necessarily society or religion. If you think thoughts which appear to you to be "bad" then thats a good thing, its a very good thing. Imagine thinking only thoughts which you considered to be good, then youd probably act on everything you thought and do in the eyes of other people "bad" things.

rather round about way of saying that bad thoughts arent in my opinion a bad thing. or if you must a "sin".