I've appreciated the points pointed (pun wanted) by Agarza's and Dottard's answers, but I believe they have omitted a pivotal topic: the 'joint witness' by God. What this means?

First of all, according God's own forma mentis, we may conclude that, before God started to use Samson as an instrument of His vengeance, against his foes (the Philistines), he was - as regards his physical power - a normal man, like me and you (I hope not like me, nevertheless...). Once, apostle Paul related what God said him in an occasion "...my strength is made perfect in (human) weakness" (2 Cor 12:9, Webster) So, also if we are - about Samson physical appearance - accustomed to see him, in a lot of pictures or movies, as an ante litteram Schwarzy - it is more probable that he seemed a normal man. When the spirit of God empowered him, he was able to perform the astonishing actions we know (without to undergo a Hulk-like transformation, necessarily).

God always avoids his servants come to confuse God's power with man's power.

There are - in the Bible - a lot of situations in which God enhanced this principle of him. One of this was the famous occasion of the 'Gideon's 300' (Judges 7). We remember that original amount of Gideon's army (ready to fight against the Midianites) was 32,000 men. But God arranged things so that that army was reduced to only 300 men. What was the reason behind that divine decision? The Bible account reports: "... lest Israel boast to Me saying, 'My own hand mad me victorious'." (Judges 7:2, Alter).

So, getting back to the point, we know that Samson prayed God to have a super-human (namely, 'miraculous') power (Judges 16:28) but nothing indicates that God was obliged to hear this prayer (as an example, God did not hear Paul's prayer about a miraculous healing for his sake, 2 Cor 12:7-9).

This is why I speak about the 'joint witness' of God. This isn't an expression ideated by me, it is biblical.

In the letter of Hebrew, the inspired writer recalled the many occasions in which God gave this witness "such as [...] healing the sick, causing the lame to walk, and raising the dead, and casting out devils, and the like; all which were for the confirmation of the Gospel preached by them: a sign, wonder, or miracle, for these signify the same thing, is a marvellous work done before men, by the power of God, to confirm a divine truth; God is the sole author of miracles [...]." (John Gill, at Heb 2:4).

Note that the pivotal long Greek term is a derivative of the verb σινεπιμαρτυρεω (sinepimartureo), that means "to unite in adding evidence" (Strong). Knowing this, read now the verse: "God bearing, besides, witness with them to it , both by signs and wonders, and various acts of power, and distributions of the Holy Spirit, according to his will?" (Heb 2:4, Darby)

It is clear, that this God's way of doing things has as purpose to make men to know two points: (1) God is the sole author of miracles (as Gill said); and (2) the human performers have His approval (as instruments of Him).

Then, if Samson did commit suicide, God so was approving his action (giving him the miraculous power to perform the temple collapse), aiding and abeting Samson in his transgression (suicide).

We may safely conclude - then - the Samson's action was not a suicide, but instead the acknowledgement of the seriousness of his sin (the disregard of Nazirite's vow of him) against God. He put his life in the hand of God, hoping He decided to forgive, whether he was spared or he died in the attempt.