It varied widely, expecting uniformity from all the various groups that are commonly classed as Vikings is simply too much. It really depends upon the time and place.

An interesting primary source for this topic is Hrafnkels Saga, written in the 10th Century. In which a man dedicated to Freyr is defeated in a duel, the temple is burned, and pledges to never sacrifice to the gods again. He eventually mellows out, finds followers who trust him, and gets revenge. Here's an English translation In Jacob Grimm's Teutonic Mythology there are several other notable cases of Kings giving up on worship of the gods altogether. The whole "rely on yourself and your trusted followers" thing was a something of a trope in Sagas.

Conversely, in the Ynglinga Saga, written in 1225, we have the story of a Swedish King who was ritually sacrificed to the Gods, something that is noted to have occurred (the sacrifice of ruling Kings, that is) on several other occasions. If the people at the time held the deities as simple mythology and tradition the whole ritual sacrifice of human beings (much less important human beings such as the Heimskringla 's story of a Swedish King who sacrificed all but one of his sons in a bid to extend his own life.

The execution of blasphemy might have been a thing, but it was hardly systematic. Without a highly developed central authority determining what is and is not blasphemy can be a challenge, much less developing the capability to execute someone over it.

I can't really comment on the exact topic at hand, but from a more theoretical perspective I think we need to be careful with applying the concept of "religious fanatic" simply because that it very difficult to do outside of the cultural context in which the term was invented. If you look at, say, Christianity, it is possible to look at the religion as separated from its wider cultural context--I view this as a grave methodological error, so I don't endorse doing so, but it is theoretically possible to treat "Christianity" as an ideological body proceeding through texts and theological arguments. It is thus possible to talk about, say, a specifically Christian action rather than an action taken by somebody who is Christian.

This becomes somewhat more complicated when we look at a society that does not have a preserved body of theological texts. Absent that, and particularly absent the sort of cultural self commentary regarding the topic we have today, all we really have are forms and actions that may look like a form of extreme religiosity, but may also be the equivalent of swearing on a Bible.