Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him. Then the LORD said to Cain, "Where is Abel your brother?"
He said, "I do not know; am I my brother's keeper?"
And the LORD said, "What have you done? The voice of your brother's blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother's blood from your hand. When you work the ground, it shall no longer yield to you its strength. You shall be a fugitive and a wanderer on the earth."
Cain said to the LORD, "My punishment is greater than I can bear. Behold, you have driven me today away from the ground, and from your face I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me."
Then the LORD said to him, "Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold."
And the LORD put a mark on Cain, lest any who found him should attack him. Then Cain went away from the presence of the LORD and settled in the land of Nod, east of Eden.
(Genesis 4:8-16 ESV)
No, horticulture is not sinful in and of itself. The Word of God does tell us what ever is not of faith is sin. (Romans 14:23) Cain has placed his trust in his own abilities above God's provision. He wanted to call the shots. He was a tiller of the earth and expected that to be his means of coming before God. God did not allow it and rejected his sacrifice. To say how he achieved this would be conjecture but it seemed apparent to Cain. When Cain took vengeance on his brother for having his sacrifice be accepted, God took his source of pride and self-sufficiency from him. This was a mercy towards Cain. Able's sacrifice was not acceptable because his skills were better or because he practiced animal husbandry, but because he offered the sacrifice by faith. Able was faithful to God's command and displayed his faith by obeying. (Hebrews 11:4) If we say we have faith, a component of that faith is that we must not simply do our own will and call it faith. This is essentially what Cain did. He wanted the favor of God. It can be deduced that he expected at least on a certain level that God would accept his sacrifice. The problem is that he did not act in faith. He did not believe God's instructions for an offering. We do not have God's instructions spelled out, but it was given prior to the sacrifice. Cain wanted to do his own thing and have God bless it. This is often the practice of the Christian, including this writer. Obedience does not equate to righteousness, but true faith is followed by obedience.
Later on the punishment for murder is later established to be execution. (Genesis 9:6) Possibly God did not establish capital punishment from the start because the problem had not arisen yet. This line of thinking is plausible, but not proven by any Scripture known to this writer. We do see that Cain is given another punishment without prior warning, so that casts doubt on the theory that God would not give a punishment without a warning. A more likely answer is that God is merciful to Cain which as the Scripture tells us is a call to repentance. (Romans 2:4) Cain did not get what he deserved but he received mercy.