Sounds like an act of Piracy on the High Seas. The vessel should be seized or sunk. It's captain and crew put on trial.
Throughout history, Russia has desired to make the Baltic Sea and the Black Sea into Russian lakes.
This incident appears to be pre-emptive sabotage against NATO Baltic communications. Finland, Germany, Lithuania, and Sweden are Baltic NATO countries.
I long suspectd that Russia had a plan to cut underwater sea cables as an asymetrical battlefront. Putin must have outsourced that to the Chi-coms for plausible deniability.