I'M ABOUT TO BECOME THE FBI YA'LL SO BUCKLE THE FUCK UP AND LET'S GO.
Before getting into this, we need to examine the body;
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Notice how there's no blood. Yet he's dead. So, it's either 1. He was strangled to death 2. He ate something poisonous 3. He was killed but the blood got cleaned up.
Now, #1. If you noticed, there's no blood on the body. So he could have easily been strangled to death.
#2 It was easy to slip something poisonous into the food. Something small that no one would notice.
#3 Now, this one seems the most least likely to happen. If there was a gunshot everyone would have heard it, or a stab Mark would yell and we would have heard it too.