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Is it bad I did this?

I was visiting my friend's house last week and I went upstairs to surprise him but when I passed the bathroom to get to his bedroom his sister's bedroom was slightly open and I heard a low moaning noise and I saw a shadow of someone.
I looked in and his sister was fully naked and standing up masturbating.
All her clothes were on the floor and she was touching herself with her two hands just and she was sweating a lot as I saw water drip off her.

His sister is in her early 20's and I find her very attractive. So I watched the whole session and waited for her to orgasm and I sneaked out as soon as was at the end of it. She was shaking a lot when she climaxed.
Am I a sex addict?
What I saw was hot.
But I feel guilty.
I would have fucked her on her bed there and then very hard.
If she wanted to.
Is it bad I did this?
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