And here we go again: it could be, if the CMDR doing the blowing up is repeatedly targeting random people.
It's a perennial problem: one person's reasoned roleplay (not griefing) appears identical to another person's psychotic roleplay (griefing disguised as roleplay "because I want to blow things up I'll say I play a psychopath, aren't I clever!") excuse appears identical to another's "let's blow people up for the lulz" (griefing.)
There are plenty of circumstances where someone could, acting fully in-character and not pretending to play a psycho, go on a killing spree of randoms, so we can't say one way or the other based on the example given. If we knew that CMDR was targeting randoms sequentially (ed) without a reason, it'd be griefing. If they were using unreasoned roleplay as an excuse it'd be griefing. Otherwise not.
blowing up a commander once (no matter HOW MANY other commanders the player blows up once) is not griefing... Yes it IS psychopathic behaviour...
To the OP I am sure it left a bitter taste in your mouth, as it does and should... Chalk it up to experience and get on with the game
It was only a viper and you had your rebuy option taken care of with the loan so its all good, you were inconvenienced the failure of 1 mission (if it was a fails on ship destruction type mission) so again no big deal... It happens, you will learn to try harder to avoid the interdiction next time I would guess... It is unfortunate that some like to play the Psychopath, its not my bag that is for sure... but meh... I have played ONLY in open since pre beta and I enjoy it... Expand your horizons, there are plenty of systems out there that have good trading, mining, bounty hunting and courrier missions available where you can make Asploads of credits