True, but if you talk with militants of Izquierda Unida specially older ones you will find that they have kind of a soviet nostalgy and are very anti NATO

You are quite spot on. The extreme left in Spain is full of Russia sympathisers. Mainly because the soviet union was the only country that offered real support in the Spanish civil war to fight off the fascist uprising and once the ww2 ended it was the main supporter of the Spanish communist party, the main opposition to Franco. Since then the Spanish left has moved away from communism but it was always tainted from a pro russian bias inherited from a pro soviet bias (like if Russia had anything in common with the soviet union). Now a days is something that only manifest in very niche ultra leftist groups. I remember a protest in support of the LPR and DPR in my city at the begging of the war with around 25 people in my city ( >150k habitats) but you just need one crazy asshole to make this kind of action.

I guess, happened two hours ago. The embassy is scheduled to make a press release. Who knows how it got through. There were no casualties and the man suffered the explosion got out in his own feet and was sent to the hospital.