I seriously doubt that Barnabas could have done away with himself. He wants to survive, and desperately, although he despises the way he must do so to go on.
And I think Barnabas, knowing that Angelique stopped Ben when she tried to stake him, figured the same thing would only happen if Joshua tried. Come to think of it, she undoubtedly would have prevented silver bullets from finding his heart, too! Once Barnabas killed her, Angelique was all about revenge.
Love, Robin