Perhaps she was upset that he only saw her as an experiment at first.
Perhaps she wanted to make sure that what happened to her would not happen again.
I don't know the answer for sure, but I guess it's more along the lines of my first explanation.
I thought it was obvious.
When Victoria asked what would have happened if Paul asked for help at the university he explained that she would have ended in a hospital behind glass where doctors would make experiments with her, she would be nothing more but a freak/monster. These lines are repeated at least two times when Victoria remembers them. Another important scene is where she finds out that nobody except Paul knows about her, she does not really exist.
So, what she did is burning all evidences that she is in fact only a year old. Nobody would believe Paul. This way she ensured that she'll never end up as a lab rat.
I think it was symbolic of what should have happened to the script.
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