In the Game of Thrones universe, getting the wrong impression of a character is quite easy. At the beginning of House of the Dragon, audiences thought that they had a handle on whom the good and bad characters were. Of course, the violent Prince Daemon Targaryen was the villain, who would use his Valyrian blade and blood-red dragon Caraxes to terrorize the kingdom after being denied heirdom by his brother, King Viserys. At the same time, other characters, like Princess Rhaneyra Targaryen and Lady Alicent Hightower, were innocent bystanders who would wind up being tragically swept up in the tribulations of the slowly collapsing Targaryen empire.
However, the balance of power quickly shifted, leaving audiences unsure of which characters to root for and against. Perhaps no character has gone through a shift as drastic as that of Lady Alicent. Initially a sweet young woman more concerned with the superficial aspects of ladyship than any practical political considerations, everything changed when King Viserys chose to marry her, thus making her the Queen. Several children and almost 13 years later, Queen Alicent has become bitter, paranoid, and full of rage, as well as she should, given the tumultuous era that House Targaryen has been slowly entering over the past decade.
The most recent episode of House of the Dragon, "Driftmark," saw her completely unhinged and revealed just how much she has changed.
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Early in House of the Dragon, Princess Rhaneyra and Lady Alicent were established as loving friends. However, when the Princess's father, King Viserys, chose to marry Alicent, their relationship begins to crumble.
Though Rhaenyra certainly has a fair few flaws, she is the main character of the series, which makes audiences view her as the more sympathetic party in most interactions, making Alicent look like the villain in their interactions by default. Over the past few episodes, she has continued to grow more and more resentful of Rhaenyra. In "The Princess and the Queen," Rhaenyra is forced to bring her newborn son to Alicent moments after his birth, and the Queen is quick to point out that the baby bears no resemblance to Rhaenyra's husband, Laenor.
Though it's easy to judge the Queen for her callousness, it's important to note that some of Rhaenyra's actions, like "coupling" with her uncle Daemon and having children with Ser Harwin Strong, a man who is not her husband, put the Kingdom at risk. Hence, her anger with Rhaenyra is somewhat justified.
Queen Alicent Attacks Lucerys Targaryen and Slices Rhaenyra 
In "Driftmark," the strained relationship between Queen Alicent and Princess Rhaneyra reared its ugly head. The episode centered around the funeral of Daemon's second wife, Laena Valeryon. Her dragon, Vhagar, was supposed to be claimed by one of her daughters, but the Queen's second son, Aemond, sneaks out during the night and claims the dragon for himself. When he returns to Driftmark, he is confronted by his cousins. In the ensuing altercation, Rhaenyra's son Lucerys slices him across the face, leaving him permanently blind in one eye.
After the attack, Queen Alicent demands retribution for her child's lost eye, even going so far as to suggest that Lucerys's eye be removed as well. When King Viserys insists that the argument ends, Queen Alicent grabs a blade and moves to attack Lucerys. Rhaenyra steps in the ways and is cut open during the struggle. This scene is the clearest instance of the Queen acting as a villain that audiences have seen in the series.
Has the Queen Already Sided Against the Crown? 
Queen Alicent's three sons are the first male children of King Viserys I, which would generally mean that the eldest son, Aegon, would be next in line to rule -- given the King's rapidly declining health, his successor may be ascending to the throne in short order.
However, King Viserys has already chosen his daughter, Princess Rhaenyra, as his successor, breaking with years of tradition by selecting a woman for the role. In the event of his death, Queen Alicent would likely become the symbolic head of the "greens," a faction that supports Aegon's ascension. Starting a civil war in the Kingdom would certainly cement Queen Alicent as a villain.
The Game of Thrones universe doesn't really have heroes and villains. In Game of Thrones, House Targaryen's own Daenerys spent most of the series as a righteous liberator before succumbing to murderous madness in the last season. Queen Alicent's actions in "Driftmark" were beyond the pale and certainly earned her temporary villain status, though she was trying to defend her child. Her story arc is far from over, and it will be interesting to see how Queen Alicent emerges from this tumultuous era in Westerosi history.