I think a bit of broader context would be useful here. Under Diocletian the Roman Empire was split into east and west, with each being controlled by an Augustus. Each Augustus had a second-in-command and successor called a Caesar. This system, the Tetrarchy, was Diocletian's main accomplishment and Constantine ripped it to bits through his repeated seizures of power. In the winter of 304/5, Diocletian suffered a debilitating illness and he decided to resign on 1st May 305. His co-Augustus Maximian also resigned. This meant that the Caesars, Galerius and Constantius (Constantine's father) became the new Augusti. Diocletian retired to his villa where he took up gardening, but remained in contact with Galerius. It was expected that Constantine would be made a Caesar, but Diocletian instead appointed Severus, most likely to prevent succession becoming hereditary like a monarchy. Constantine fled to his father, who promptly died in 306, leaving Constantine as de facto leader in the west. Galerius, probably in consultation with Diocletian, agreed to make Constantine Caesar of the West and elevate Severus to Augustus. This did not sit well with Maxentius, the son of Maximian, who thought he was a better candidate. Maxentius declared himself Augustus of the West, and Severus' legions defected to him. Constantine now had an excuse to size total power in the west. Maximian attempted to reclaim his former position of Augustus of the West, but he had no legitimacy so did not get very far. As of 308 there were 3 people claiming to be the Augustus of the West.

In utter despair of the situation, Galerius - Augustus of the East - called the Counci of Carnuntum to sort everything out and Diocletian was invited, which suggests that up until that point Diocletian still had an active hand in Roman politics. Unfortunately, the letters between Galerius and Diocletian do not survive so we do not know what their opinions were. Maximian was forced to formally resign again and Constantine was demoted to Caesar again, Severus had died. The council was dominated by Galerius and Diocletian, but the council seems to have taken a toll on Diocletian's mental health. Many implored Diocletian to reclaim power, but at the end of the council he allegedly responded "If you could show the cabbages that I planted with my own hands to your emperor, he wouldn't dare suggest that I replace the peace and happiness of this place with the storms of a never-satisfied greed.". This meant that Diocletian was done with politics, and simply wished to go home to his plants. From then on he totally withdrew from public life.

Neither Constantine or Maximian accepted their demotions and they fought a civil war. Galerius was suffering from a debilitating illness which eventually killed him in 311, so he could not stop them. There was a power struggle between two men named Maximinius and Licinius to be the new Augustus of the East. Licinius was at a disadvantage, so he formed an alliance with Constantine. When Constantine defeated Maxentius at the Battle of Milvian Bridge in October of 312, a damnatio memoriae was issued against Maximian, which meant that all images and mention of him were to be erased. During the Tetrarchy, the 4 emperors had always been shown embracing one another, so destroying statues of Maximian also meant destroying almost all the statues of Diocletian, Constantius, and Galerius. This included the statues at Diocletian's own villa. He also probably feared personal reprisal from Constantine, given that Diocletian had constantly stood in Constantine's way. To top it off, Diocletian then heard of the alliance between Constantine and Licinius, and therefore saw the usurper be legitimised. According to Lactantius' De Mortibus Persecutorum, Diocletian committed suicide a few days later. He may have just been ill; Lactantius wasn't present, but the fact that suicide was seen as the logical and fitting end to his life speaks volumes about Diocletian's state of mind in his last days.

So what did Diocletian think of Constantine? He was one of the men who ruined everything. Diocletian had taken great care and effort to create a political system that he thought would finally bring stability after the Third Century Crisis. It had worked, but Constantine, Maximian, and Maxentius had totally destroyed that system. Constantine not only wanted to rule, but destroy the memory of the Tetrarchy that had hindered his rise to power. It is therefore hard to believe that Diocletian died with anything other than a deep hatred and despair for Constantine.

Secondary sources:

Nakamura, Byron J. "When did Diocletian die? New evidence for an old problem." Classical Philology 98.3 (2003): 283-289.

Barnes, Timothy D. "Maxentius and Diocletian." Classical Philology 105.3 (2010): 318-322.

Mitchell, Stephen. A history of the later Roman Empire, AD 284-641. John Wiley & Sons, 2014.