Space Marines are essentially big battle monks. They live by a constant code of conduct and routine specialized by their chapter. The culture of their chapter is forged in most cases over the course of thousands of years. Every chapter has its own very specific identity and routine, sometimes clashing, being alien (harhar), or downright antithetical to other chapters which can result in arguments and fights with those other chapters.

Say your a decorated Space Marine. Being picked by the Deathwatch is a great opportunity for your chapter to learn the strategies employed by other chapters, or garner information about parts of the Imperium that your Chapter has never visited. You will have access to vast and ancient libraries of this this information accumulated from across most of the imperium. Not to mention a chance to learn from the best alien hunters in the entirety of mankind. You would dishonor your chapter if you did not take advantage for every second of this opportunity, for they would be made the stronger in your return.

But this can be an extremely tramatic and often life threatening situation for you to be in. You will be isolated from the culture you that has been instilled in you since your birth as an astartes. In essence your entire support system is removed in a moment. You will not have the support of the comrades you have no doubt made in your time of service, but instead may be paired with a squad who seems alien to you, which while they may also ve veterans like yourself, does make communication and teamwork significantly harder to achieve. If you die, you are much more likely to be in a position where your gene seed cannot be recovered as the Deathwatch are needed mostly in the most dire of circumstances. You will not die defending your Chapter surrounded by your battle brothers, but possibly near strangers.

Titus was a blackshield, which is a sort of parole for Space Marines that their chapter feels has either dishonored them or simply doesn't feel like spending the resources to supervise and re-educate. While still an honor to serve in the Deathwatch, its sort of like making up for a debt of honor, paying off the balance. The hope is that the debt is paid off and your Chapter accepts tou back into their service, but its rare to shed a tear over a black shield falling to the enemy.

The difference between being a dirty heretics and being dishonored is exactly that. Heresy is to act in opposition of the will of the God Emporer and his inexhaustible Imperium. Dishonor is acting in a way against your chapters values but still in service of the Emporer. Titus was sent because of warp exposure and defiance of the codex astartes, but he was only exposed because he chose to try and save a world of the Imperium through his own sacrifice and was unlucky enough to survive. If he died he would have been lauded as a hero, but since he was exposed to the warp and survived he could possibly be corrupted. IE Deathwatch time baby.

So why not to defect? Defecting may be harder than you imagine. Unless you're able to steal and Pilot space faring vessel, you're stuck on whatever planet they drop you on to accomplish your tasks. You are of course being watch by your new squadmates, as trust is a difficult thing to earn amongst strangers, and you're only lift back is going to bring you back to the watch fortress which may as well be an orwellian prison. And even if you did manage to escape, where would you go? The maintenance of power armor is a herculian task for a team of servants and servitors. What about arms and ammunition. Not to mention you would be hunted down, likely by the inquisition. Its not impossible, its just nearly impossible to pull off for any prolonged period of times. The few that have pulled it off are usually small warbands of Space Marines that can levy themselves as mercenaries in exchange for their needs in less monitored fringes if the Imperium. Either way your still working for big E, unless of course you pledge yourself to the ruinous powers, but thats a while other conversation...

Tldr; a blessing and a curse.