Funnily enough I answered another question about Venetian law yesterday! Check that one out for a fuller discussion of Venetian law in general. But in summary, in the year 1000 they didn't have a written law code yet. Venice didn't use Roman law or Germanic law, and they didn't write down their own laws until the 13th century. From the 13th century onward, the punishment for murder was quite severe - depending on the circumstances of the case, punishment for murder could consist of being paraded up and down the grand canal, maybe mutilation of some kind (cutting off the hand you used to commit the crime), and perhaps execution after that.

In 1000 we don't really know what would have happened. There were no separate law courts at that point. The city was still relatively small then, so people would bring all of their complaints (from property disputes to the most terrible crimes) to the Doge, and the Doge would settle all their cases personally. So your punishment for killing a fellow Venetian would be entirely up to the Doge's discretion. Presumably the later medieval/early modern punishment was a codification of earlier customs, so maybe there would be a public mutilation/execution in the year 1000 as well, but it's not really clear.