And why was Samson preparing to marry one of these enemies ?
That was part of the man's weakness.
You DO understand that many people God used in the Bible had WEAKNESSES and made errors, I hope?
Noah getting drunk and being found naked in his tent;
Abraham lying about his wife that she was his sister ... TWICE;
Isaac blessing the wrong son;
Jacob deceiving his brother;
David stealing someone's wife and murdering her husband;
Samson wanting to marry a Philistine woman and latter going to the harlot
Delilah.
Peter denying his Master Jesus three times under pressure.
You do understand that the only PERFECT servant of God was His Son Jesus Christ? As you read the Bible you should not be surprised if occasionally a servant of God is recorded as doing something which he or she should not have done.
And why would they have any grievance against these people who God had decreed to oppress them.
The oppression of the Philistines upon the Hebrews you should consider as God's discipline of a negative sort.
I may commit a crime and be turned over to the jailors. That being turned over to jailors may be a discipline assigned to me by God. When one is allowed by God to fall under such circumstances there is no gaurantee that one will not have bad experiences and resentment against those into whose hands God has allowed you to fall.
On the other hand the prophet
Daniel had a somewhat more wise view of the Babylonian Captivity. So with different servants of God you do see sometimes different reactions to their circumstances.
The Philistines were doing the direct will and judgment of the God of Israel. Seems rather contradictory, don't you think
Seems paradoxical. But it is realistic. Yes, indirectly the Philistines were being used to turn the Hebrews back to God. They might have even had glee to hear that themselves. But actually the Philistine attitude was not in the least spiritual or godly. It was totally selfish and cruel.
If you continue to read the Bible you will see a number of times after God used some nation to discipline another nation, He then turns to the disciplining nation and rebukes them for going too far.
When we get to the perfect Son of God, Jesus, He did teach His disciples to love their enemies.
" You have heard that it was said, "You shall love your neighbor and hate your enemy." But I say to you Love your enemies, and pray for those who persecute you,
So that you may become sons of your Father who is in the heavens, because He causes His sun to rise on the evil and the good and sends rain on the just and the unjust.
For if you love those who love you, what reward do you have? Do not even the tax collectors do the same?
And if you greet only your brothers, what better thing are you doing? Do not even the Gentiles do the same?
You therefore shall be perfect as your heavenly Father is perfect? (Matthew 5:43-48)