Don't know if it's already been said, but any parallel to a human 'giving a warning' before smacking you in the face is null and void.
If two humans encounter each other there are basic, almost universal, social and anthropological rules which govern how that will unfold. If one person is threatening then it's obvious by tone of voice, body language and so on (even without a shared language); and if the other person doesn't like it, then there are other signals which broadcast that. Thus, two people who can't converse can meet and, generally, be comfortable knowing whether the other person is being threatening if they get close; or is taking offence at the other's proximity, etc.
So when somebody breaks those rules - e.g. attacking without being provoked - it enables us to justify retaliation, under a new social and moral ruleset known as 'self-defence'.
These are aliens - there is no way of knowing the true extent to which our presence is threatening, or intimidating or whatever. After all, some grown human adults swat at moths and run away screaming.
Even more importantly, there are some of us who do that; then others who are ambivalent to moths, still others with a pathological hate and others who catch moths in order to study them.
In space, we have navy ships, pirates, explorers, scientists, traders, cruise liners etc. Some will attack first - either due to that same hatred, or through pure fear, some because they want cargo - others will study and others will immediately flee.
I think these Thargoids are exhibiting exactly the same variance and, therefore, to label the whole lot as either 'peaceful' or 'aggressive' is too simplistic; just as it is with us.
The fact that these Thargoid vessels are practically indestructible at the moment and inherently threatening because of their size and alien behaviour does not make them all aggressive. Based on our social rules, yes - but not necessarily in theirs.
We have to establish the baseline for their social norms so that we can see how they fit with ours; to reach some kind of understanding as to what it is that constitutes a threat to them so that those of us who do not wish to end up in a shooting war can avoid it.
However - since there will be some of them who, just like us, have a pathological hate for anything alien and who will attack first, regardless - what ultimately governs whether that leads to inter-species war (and likely destruction for one or other or both) is whether we respectively want to allow the actions of the aggressive minorities to speak for the whole species.
And that is something which humanity has consistently found a challenge, from interactions between individuals, families, villages, counties, states, nations, groups of nations, sexes and those with different facial features , colours of skin or physical and/or mental disabilities.
Just look at the last couple of years in RL.