As you can imagine it's always a multi-faceted affair when it comes to politics. After Hulagu sacked Baghdad in 1258 he annihilated the Muslim population of the city and tortured Al-Mustasim, the Caliph at the time. He did however spare the Christian population of the city because of his sympathy for them. There are multiple reasons why he spared them; firstly, one of his commanders, the Naiman known as Kitbuqa who seemed to have won Hulagu over with his loyalty was himself a Nestorian Christian. Secondly, Hulagu had many auxiliary forces whom were of Armenian or Georgian descent. And lastly, and probably most importantly his wife. The Armenian chronicler Kirakos Gandzaketsi wrote this:

At the capture of Baghdad, Hulagu's wife Doqutz Khatun, who was a Nestorian, spoke on behalf of Nestorians and on behalf of Christians of any other confession, and interceded for their lives. Hulagu not only spared them, but allowed them to keep all of their belongings.

The otherwise savage Mongols were now seen as saviors and protectors of the Christians and Kirakos further writes about how Hulagu is to thank for the end of Muslim tyranny that had lasted nearly 7 centuries. But make no mistake about it, it was the Khatun who should have been thanked for this; Doquz was constantly consulted by Hulagu, not only because she was his wife but she must've been diplomatically remarkable as the Great Khan Mongke (Hulagu's brother that was currently ruling the Empire) had advised Hulagu to look to her for diplomatic advice.

Note: Khatun is the female title equivalent to Khan

Here is where we come to the 2 possible perspectives. Was Berke A: angry at his cousin for the mistreatment of Muslims, or B: did he view his cousin as a usurper of his lands?

Let's start with A:

When the Great Khan Mongke died, Hulagu needed to return per Mongol tradition to the Kurultai to partake in the elections of the next Great Khan and thus, his expansion into Muslim Syria was now halted. He left his loyal commander Kitbuqa in charge of the region where he looked to use his Christianity to appeal to the Franks on the Levant. Behemond VI was all for allying himself with Kitbuqa, but many of the Barons such as the Baron of Acre viewed the Mongols as savages worse than the Muslims. Kitbuqa was left with a small force to fight against the Mamlukes who were also looking to conquer the Levant and Syria. The result was the famous battle of Ain Jalut where Baibars and Qutuz will defeat Kitbuqa and his allies. For the Mongols the situation turned grim pretty fast as the vast Empire was subject to factionalism, Ariq Boqa and Kublai had both broken tradition by claiming themselves as the new Great Khan. Hulagu supported Kublai's faction while Berke supported Ariq's. Rashid al-Din quotes Berke when speaking of the Khan of Persia (Hulagu):

He has sacked all the cities of the Muslims and, without consulting his kinsmen, has brought death to the Caliph. With the help of Allah, I shall call him into account for the innocent blood he has spilled.

Based on this account we can see that Berke demands retribution for the discriminatory killing of Muslims by Hulagu. It almost gives the sense that Berke who himself was Muslim, had more affection towards his fellow believers than Hulagu had sympathy for the Christians. Berke, being in charge of the Khanate of the Kipchaks, or as we know it in the West, the Golden Horde, allied himself with the Mamluke Sultan Baibars who was supposedly of Kipchak origin and also had recently fought against one of Hulagu's commanders. They formed an alliance and Berke declared war on Hulagu and these civil wars between the factions is marked as the end of the Mongol Conquests. What's interesting though is that the result was somewhat favorable for the Persian Khan because due to this war he was able to fully unify Persia under his rule. Kublai Khan will eventually win his side of the conflict and award the title of Ilkhan to our friend Hulagu here; the title roughly translates to 'lesser' Khan which sounds like a bad deal but it really wasn't because for the Mongols loyalty was huge.

Hulagu was given the nickname "scourge of God" by the Muslims and though he did commit atrocities he was at least respectful and an admirer of Persian culture. Not sure this is a redeeming factor, but it's there.

B.

I mentioned that Berke "supported" Ariq in the war against Kublai. Well, I said he sided with his faction, but that's about it. He didn't send any military help as he not only feuded with the Persian Khan but also with the House of Chagatai in Central Asia. He fought against Alghu, the Khan of the Chagatai Khanate in Central Asia where he lost control of some important regions like Khwarezm and also, Alghu took Otrar from Berke's brother Orda, a very important trade city. He couldn't do much about it as he was fighting in the Caucuses against Hulagu and this is where the political beef comes from. Hulagu had occupied modern day Azerbaijan and it gave Berke the feeling that Hulagu wanted to usurp his territories for himself. This is supposedly one of the reasons that Berke allied himself with the Mamlukes and some claim that Islam was used as an excuse by Berke, but I don't see how both reasons could be both together a combination of reasons for Berke.

The real winner here of course, without a shadow of a doubt in my mind, is Baibars. The civil war kept Hulagu busy so he and Qutuz were left to fight a small force of Mongols and their auxiliary forces at Ain Jalut, I don't see how the Mamlukes would have stood a chance against the full force of Hulagu's forces. Baibars was also able to take control of Syria because of this and lastly, the trade you're talking about is a result of the alliance between the Golden Horde and the Mamluke Sultanate. If you remember I mentioned that Baibars is thought to have been of Kipchak origin, so during his alliance he sent his emissaries to the Crimea, an area that was inhabited by the Kipchaks, and he was able to appeal to the Kipchaks and recruit them for himself. This alliance with the Golden Horde was fantastic for Baibars as the squabbling between Berke and Hulagu became more and more petty. Hulagu was angry he got injured in a battle and executed all Kipchaks in Persia, likewise Berke had Persian merchants in the Golden Horde executed, the battles had caused massive losses on both sides and Hulagu was never able to avenge Ain Jalut.

The Empire of the Steppes: A History of Central Asia by Rene Grousset translated by Naomi Walford

The Mongols and the West by Peter Jackson