is it possible to be a communist and a terrorist

Franco thought so. Thousands of british Communists went over and were regarded by Franco the same as we regard ISIS. That is why the Brirish security forces distrusted and kept tabs on them.
Even the lefty rag the gruniard agrees:
Today they would be arrested under section five of the Terrorism Act 2006. If convicted of fighting abroad with a "political, ideological, religious or racial motive" – a charge they would find hard to contest – they would face a maximum sentence of life in prison. That they were fighting to defend an elected government against a fascist rebellion would have no bearing on the case. They would go down as terrorists.(and quite rightly so).

https://www.theguardian.com/commentisfree/2014/feb/10/orwell-hero-terrorism-syria-british-fighters-damned