Your example is not ungrammatical but semantically dubious. If we change "rudo" to "descortés" or "de mala educación" as I suggested in my comment on your question and "comenzó" to "había comenzado" (perhaps even in English "being that the event had started an hour before" would be more appropriate), the result would be somewhat better:
Era de mala educación acabar de llegar, siendo que el evento había comenzado hacía una hora.
I'd say that the sentence above is unlikely to occur in Spanish, because -- as I also said in my comment above -- the verb phrase "acabar de llegar" is generally neutral, and your sentence seems to attribute a negative meaning to it, as if it meant: "llegar tan tarde".
If you need to use the infinitive "acabar" rather than a tensed form of that verb, a more reasonable example could be:
Era raro acabar de llegar y ya sentirse como en casa. (It was odd to have arrived so recently and already feel at home.)
Notice that "arriving recently" does not have the negative connotation of "arriving late".