I’m not a lawyer, but if I wasn’t planning to do anything related to the other ‘dedsec’, I wouldn’t even consider what the owner of that dedsec would think.
Businesses have the same name ALL THE TIME.
Unless you’re trying to piggyback on or undermine that other dedsec, I, the non-lawyer, can’t imagine how they’d have any standing to raise a concern.
Whatever you’re doing must have led you to the dedsec name (I assume it’s a website for dog-eat-dog security), there’s just no way in my (non-lawyer) mind that EA or Ubisoft has any right to stop you from saving dogs from being eaten.
I (the non-lawyer) would 100% not pay a lawyer for their opinion on this.