It looks like Oliver Queen (Stephen Amell) will have some trouble handling relationships with the people close to him in the upcoming episode of "Arrow" season 6. In episode 18, titled "Fundamentals," the dark thoughts that have been troubling Oliver's mind will take a toll on his behavior, lashing out all his frustrations on Felicity (Emily Bett Rickards) and William (Jack Moore).

According to the official synopsis for "Arrow" season 6 episode 18, Arrow will begin to question his worth as a father, a mayor of Star City, as well as being the caped hero that protects the citizens. His recent failures in every aspect of his life will push him to his limits, and the viewers could see the character having trouble with his rage.

The first in his list of disappointments is his recent fallout with John Diggle (David Ramsey), a person that had always stood with him through thick and thin. Diggle is the latest member of the already divided Team Arrow to make an exit, and it should be expected that Oliver will be at his most vulnerable state at this point after losing his best friend.

Also, there is Diaz (Kirk Acevedo) who has gained more control in the Star City police department, giving him power to further antagonize Oliver. With only a few people on his side, it will be more difficult for Oliver to resist Diaz's efforts in destabilizing his position as mayor.

As darkness now envelops his mind, Oliver will strain his relationship with the people around him, most especially the remaining members of his team. His relationship with Felicity will be affected as well, even as the latter only wants what is best for him.

In the end, Oliver is no longer expecting any help from Team Arrow as the episode 18 promo shows the hero confronting Diaz all by himself. Things could get bad as Oliver has no one to watch his back, knowing how treacherous Diaz can be.

"Arrow" season 6 episode 18 will air on Thursday, April 12 at 9 p.m. EST on The CW.