In the first murder, he poisons the victim. On the second attempt, he tries to send a bomb to the victim.
We can arrive at this answer because:
Lord Arthur receives a prophecy that he will be an assassin.
He is engaged and eager for marriage, but he is afraid to marry without having completed his destiny and therefore decides to become a murderer as soon as possible.
So he tries to poison his aunt and send dynamite to a university dean.
Both attempts fail and he is unable to murder either victim.
With this, we can see the theme that we should not try to manipulate fate, we should just wait for it to happen at the right time.
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