The main definition for "pirate" is somebody who plunders by sea (the "stand and deliver" reference earlier made me chuckle, as it's more land -based thievery). So, Vikings, yes, were included in that number, most of the time, but mainly, as long as there have been boats and harbour towns, there have been people to rob them. Julius Caesar, as a youth, was kidnapped by Mediterranean pirates long before the Vikings had their identity, for instance.
The main difference was that in Roman and Viking times, the act itself was not considered the crime, but against whom you committed it. Most clear-cut of all is that a Roman citizen should, in theory, have been able to trek from one side of the empire to the other without being molested, but there were myriad, smaller, sometimes-shifting non-aggression pacts between various demesnes throughout history.
Soca lot of Vikings did, yes, settle down and become tradesmen, farmers, etc. But some never did. Some Vikings went on to become mercenary soldiers, some even protecting the last of the Roman rulers towards the end of the Empire.
What most kids play at, most fancy-dress hirers reckon they're doing (they're not, don't even get me started), the "aaarrrrr" Cornish accent, is late 1600s through 1700s.
In this respect, it's worth noting:
-
Again, it depends on who's doing the thieving and against whom. What the Spanish called pirates, QE1 called explorers and privateers, because they worked for her (and, crucially, she wasn't going to care about no Papal bull forbidding anybody BUT Spain from trading with the New World!). Some perfectly otherwise-law-abiding settlers in the New World, by the very fact of selling their goods to an English ship instead of the Spanish monopoly, were committing acts of piracy.
-
But yes, things could get nasty. Pirates would steal food and water from each other if they were low on supplies, leaving the other guys to die of thirst or starve.
-
Even the "legitimate" naval forces looted. It was how they made the bulk of their money, and was a great incentive for the crew.
-
Pirates could and would retire when they'd made enough money. The main difference between this time and Viking times was that ex-(golden-age)pirates were keener to hide themselves. There's more than a few "great families" of America whose first ancestor was suspiciously rich and scarred. Strange "retired sea captains" were constantly showing up in obscure English towns, buying the local big house and settling down to become local squires.
So, yeah, not as clear-cut as "bad guys", but sometimes even nastier than people imagine.
There's an hilarious Robot Chicken sketch of a gal wanting her boyfriend to dress up like Captain Jack Sparrow, but draws the line when he does the full "smellovision" experience. I can't link, but worth a YouTube search, for some light relief from my history soapbox! 