God is certainly and infinitely merciful when it comes to relation with Him, but not at all merciful if you behave ill towards His Bhakt (devotes). Throughout Indian epics, Puraan, wherever anyone had sustained enmity towards God, he has been rewarded in proportion to how much he had remembered God. God being all love and merciful, does not discriminate against you, even if you remember him even with hatred. So we see instances of Raavan, Kans, Shishupaal etc who were killed by God Himself to grant them Mukti as a fruit of their constant remembrance of God.
On the other hand, if someone troubles God's devotees or God's beloved, God punishes him in the most severe manner. This we see in Kaurav, Karn, Ashwatthaamaa and others who hated Paandav, who were Krishn's devotees, they all were punished or killed in various manners, but none was killed by the Lord Himself. Only Bheeshm, who was fighting from the side of Kaurav and did not hate Krishn or His devotees (the Paandav) was given an opportunity to be killed by the Lord when He raised his Sudarshan Chakra to kill him, although He did not kill him because it was not in His plan, (it was against His own vow. For God protecting the vow of his devotee, like Arjun, was more important than protecting his own vow made in any incarnation. But, Bheeshm's ego of being a warrior was so great in comparison to the force with which Krishn broke His vow. Also Krishn might have had a purpose behind not killing Bheeshm at that time as he became instrumental in teaching Dharm to Yudhishthir and others after the war was over.
There is another example of how God punished the enemies of His devotees - Lord Vishnu's disc followed Rishi Durvaasaa to kill him because the Rishi had cursed the Lord's devotee, the King Ambareesh, who was observing Ekaadashee Vrat in devotion to Lord Visnhu.
I am sure if you dig into Indian epics and even later history of various saints, you will find similar examples. Moral of the story - hate the Lord by all your means, but as long as you remember God because even by hating Him also you remember Him, you will benefit - but never hate or trouble his devotees in whatever form.
There are other examples of hatred towards Vishnu - of Jaya and Vijaya, the two doorkeepers of Vaikunth, who when cursed by Sanakaadi Muni did not want to live without Vishnu for long time. Given an option that 'whether they wanted to live as His Bhakt for 7 lives or as enemy for 3 lives, they opted for 3 lives as enemy. Hiranyakashyap, Raavan and Shishupaal, all three hated Vishnu like anything, and all of them were killed by Vishnu Himself to give them good Gati, whhile the enemies of His Bhakt were not touched by Him.
So you may hate the God as much as you like, God Himself has to punish you; but never hate His Bhakt, He will never spare you for that.