Although he did make politically motivated comments and shouted "Allahu Akbar" and "I love God. I love Allah" - which would be an odd thing for a non-Muslim to say - mentally unstable people do strange things, so who can tell. It may well transpire that it is proved that he did just have psychological issues.
However we should be wary of this initial line of reasoning. Robert Spencer notes at least two dozen previous attacks by Muslims in North America and Europe who were initially thought to have psychological issues (I encourage you to scan the list).
First, what if such a trend may indicate that Muslims with mental problems are more prone to act out violently and attribute such violence to serving their god? If that's the case then shouldn't we ask why this repeats within this particular religion? Where are all the mentally unstable Christians crying out "Jesus is Lord" as they stab people to death? There are far more Christians in the West than Muslims, but we don't seem to hear of many Christians doing this.
Second, what about us? I think also that we need to be aware that our rush to point to mental issues is a natural response to the horror of the attacks. Even though lone wolf attacks are in keeping with the doctrine of jihad and encouraged by the Islamic State and other such groups, we automatically attribute them to someone having mental issues due to the grisly nature of the attacks. But could this be a genuine weakness in our current Western epistemology that we should become aware of?
Islam is a very different religion to Christianity. The central figure, Muhammad, married a six year-old girl and performed torture, rape, assassinations and led war parties - all according to Islamic sacred scripture. Someone seeking to emulate such a person may initially seem mad from our modern point of view, mightn't they?