I let Tom down.
Would “Jag besvek Tom.” also be right? Could I add it as an alternative answer?
As I understand it, “att svika” can also mean “to betray”. But if the meaning is “to disappoint”, is there a difference between att svika and att besvika?
There is no verb “besvika”. 
“To betray” as in “betray one’s country”, you have “förråda”. For “to disappoint”, you can also use “göra besviken”.
Thank you for the explanation!
I had never seen the verb “att besvika” before, I just knew the adjective “besviken” and thus assumed that there is a corresponding verb for it, too.
Googling then showed me these two links:
I don’t understand half of the svenska.se page though.
Well, “besvika” is not in use in modern Swedish. I didn’t even know it existed.
The text was published 120 years ago.
I trust a native speaker’s assessment and your’s in particular. Thank you for the context. 