It was so with Seneca.

Circumstances--possibly a genuine detestation of Messalina's exceptional infamy--seem to have thrown him among the partisans of her rivals.

Messalina was only waiting her opportunity to strike a blow.

Julia, possibly as being the younger and the less powerful of the two sisters, was marked out as the first victim, and the opportunity seemed a favourable one for involving Seneca in her ruin.
His enormous wealth, his high reputation, his splendid abilities, made him a formidable opponent to the Empress, and a valuable ally to her rivals.

It was determined to get rid of both by a single scheme.